The Due Diligence Timeline: A Realistic Week-by-Week Timetable

A mid-market deal (roughly £50M to £500M enterprise value) typically budgets six to twelve weeks for confirmatory due diligence, and a bank-run auction with a tight signing date compresses that to three or four. Those numbers hold until they don't. Analysis of more than 900 global transactions by Bayes Business School, published with SS&C Intralinks in 2024, found the average pre-announcement due diligence period had stretched to 203 days by 2023, up from 124 days in 2014. The deal timetable a team writes in week one is a plan, not a fact.

That gap between the plan and what actually happens is the part most teams under-manage. A due diligence timeline slipping by two or three weeks feels like a scheduling problem for the deal lead to absorb. It is not. Every advisor still on the clock during that slippage is a time-and-materials cost centre that keeps accruing whether or not the extra weeks produce anything new.

How Long Is a Typical Due Diligence Timeline?#

A due diligence timeline depends on deal size, target type, and how the fee arrangement was struck, and the range is wide enough that a single benchmark number is close to useless on its own. A clean, small owner-managed business with a responsive management team can clear diligence in four to six weeks. A £50M-£500M mid-market deal with several workstreams (financial, legal, tax, commercial) typically runs eight to twelve weeks. Large, cross-border, or regulated transactions routinely run three to six months once antitrust or sector approvals enter the critical path, and at the very top of the market, Bain & Company's 2026 M&A Midyear Report found deals above $10 billion take roughly seven months from announcement to close before integration even begins. That figure is a megadeal statistic, not a mid-market one; it says little about a £100M carve-out, but it shows the same lengthening pattern at the top of the market that the Bayes/SS&C data shows across the middle. "The great M&A rebound of 2025 was no one-off blip, and the strategic logic driving it has only intensified," said Suzanne Kumar, executive vice president of Bain & Company's global M&A practice, in the same report, a reminder that deal volume is not slowing even as each individual deal takes longer to clear.

The Bayes/SS&C Intralinks data adds a second useful cut: target type. A private target's due diligence period averaged 234 days, against 125 days for a public target, nearly double, because private deals lack the disclosure discipline that public-company reporting and regulatory scrutiny impose. "Due diligence periods have gotten longer, and the process has gotten more complex, often requiring more documentation," said Ken Bisconti, Co-Head of SS&C Intralinks, describing the same dataset. That figure measures the window from data room opening to public announcement, so it excludes any confirmatory or bring-down work that continues after signing, and it is a global sample rather than a UK-specific one. The same study found something worth sitting with: deals with a "medium-length" diligence period of around 139 days were more likely to actually complete, and closed faster once terms were struck (about 104 days from that point), than deals that were either rushed or dragged out. That correlation could run the other way, too; a deal that was always going to be straightforward might both take a moderate amount of time and complete easily, rather than the moderate timetable causing the completion. Still, there is a sweet spot, and it is not the shortest possible schedule.

What Does a Realistic Diligence Timetable Look Like Week by Week?#

For a typical mid-market deal, a defensible timetable breaks into five phases rather than a single undifferentiated block of "diligence weeks."

Smaller deals compress this to four to six weeks by running phases in parallel rather than sequence; large or cross-border deals stretch each phase and add a regulatory-clearance track that can run independently of the commercial timetable. The point of writing the timetable out by phase, rather than as a single end date, is that it gives a deal lead a place to notice slippage early. A deal that is still in "first-pass review" in week seven, when the plan called for "deep dive," has already lost two weeks and the invoice has not caught up yet.

Why Do Diligence Timelines Slip?#

Diligence timelines slip most often because the scope agreed at kickoff does not match what the data room actually contains, and every extra week the timetable absorbs while advisors chase that gap keeps their meter running. A messy or incomplete data room is the most common trigger. So is a target that turns out to have more entities, jurisdictions, or unresolved legal matters than the engagement letter anticipated. Management availability is a quieter cause: a diligence phase can stall for a week simply waiting on the right person to answer a Q&A thread, and that week is invisible in a spreadsheet until the invoice reflects it.

This is where the scope creep mechanism and the timeline mechanism are the same problem viewed from two angles. Scope creep is what happens when the work expands beyond what was priced; timeline slippage is what that expansion looks like on a calendar. A team that only tracks advisor fee estimates against actuals at the end of an engagement sees the cost consequence of slippage a month after the scheduling consequence was already visible to anyone watching the timetable.

Every Week of Slippage Is a Cost Event#

A due diligence schedule is usually treated as a project-management artefact, something the deal lead updates in a status deck, separate from the budget the finance function is tracking. That separation is the mistake. Most diligence advisors are still engaged on time-and-materials terms for at least part of the scope, meaning the fee accrues against hours worked, not against a fixed milestone. When the timetable slips two weeks, a time-and-materials advisor does not pause and wait for the deal to catch up. Staff stay allocated, the clock keeps running, and the run-rate that was priced for a ten-week engagement quietly becomes a twelve-week bill.

This is a narrower, sharper version of the wider due diligence cost overrun problem: the driver is not that advisors are expensive, it is that nobody is pricing the drift while it happens. A slipped timetable is a leading indicator of an overrun, visible weeks before the invoice confirms it, if anyone is reading the schedule that way. Most deal teams aren't. They read the timetable to know when the SPA will be ready to sign, not to ask what an extra fortnight of legal and financial review is costing against the original estimate.

How Do You Read a Timetable as a Budget Instrument?#

Reading a timetable as a budget instrument means treating each phase boundary as a checkpoint where scope, schedule, and spend are all reviewed together, not just schedule.

None of this eliminates slippage. Diligence surfaces genuine unknowns, and a target that turns out to be messier than expected sometimes deserves an extra fortnight of scrutiny rather than a rushed sign-off. Cutting a diligence period short correlates with worse outcomes, not better ones, in the Bayes/SS&C Intralinks data. The goal is not a shorter timetable. It is a timetable where slippage triggers a cost conversation on the week it happens, rather than a surprise on the invoice a month later. A deal lead who can point to the exact phase where the schedule and the spend diverged has a very different conversation with the IC than one who can only explain, after the fact, why the final bill was 20% over.

Frequently Asked Questions#

How long does due diligence take on a typical deal?#

It depends heavily on size and target type. A small deal often clears in four to six weeks, a £50M-£500M mid-market deal typically runs eight to twelve weeks, and large or cross-border deals can run three to six months. The SS&C Intralinks/Bayes 2024 study put the average pre-announcement period at 203 days across 900+ deals studied between 2013 and 2023.

What is a realistic diligence schedule for a mid-market deal?#

A common structure runs kickoff and data room build in weeks one and two, first-pass review in weeks three to five, deep dive and management sessions in weeks six to eight, reporting and SPA negotiation overlapping in weeks nine and ten, and confirmatory close-out in weeks eleven and twelve. Smaller deals compress these phases in parallel rather than sequence.

Why does a due diligence timeline slip?#

Timelines most often slip because the data room turns out messier or the target more complex than the scope assumed at kickoff, or because management availability stalls a workstream's Q&A. Since most advisors are engaged partly on time-and-materials terms, a slipping timetable also means the fee is accruing beyond the original estimate, often before anyone reprices it.

Does a longer due diligence period mean a safer deal?#

Not necessarily. The Bayes/SS&C Intralinks research found deals with a medium-length diligence period, around 139 days, were more likely to actually complete and closed faster once terms were struck than deals that were either rushed or dragged out unusually long. Neither extreme is the goal; matching the period to the target's actual complexity is.

How do you catch diligence timeline slippage before it becomes a cost overrun?#

Track spend against scope by workstream and phase, not just against a single end-of-engagement total, so a missed phase boundary is visible in the same week it happens rather than at the next status update. Some deal teams do this in a shared spreadsheet; others use a tool like Advilink to see committed and actual spend against the agreed scope as the timetable moves, rather than reconstructing it after the invoice arrives.

Sources#

1. Private Markets Due Diligence Places Outsized Demands on Dealmakers, SS&C Intralinks / Bayes Business School M&A Research Centre, 2024. Analysis of 900+ global M&A transactions, 2013-2023.
2. Global M&A momentum builds in 2026 as megadeals surge, Bain & Company, 2026 M&A Midyear Report, June 2026.