The honest answer has 3 parts, because a cost segregation project is really 3 steps with different clocks: screening, the engineering study itself, and the tax work that puts the study on a return.
Step 1: the screen, same day
A screening estimate models the likely reclassification percentage and first-year benefit from the address, purchase price, and property type. Amadae's cost segregation screen inside its savings playbook does this in minutes. It is explicitly not an engineering study; its job is to tell you whether the study fee is worth paying before you spend anything.
Step 2: the engineering study, typically 4 to 8 weeks
Independent engineering firms generally deliver a full study in 4 to 8 weeks from signed engagement, though timelines vary by firm and season. The drivers are document collection, the inspection, and report preparation. A virtual inspection using video, photos, and plans usually schedules faster than an on-site visit. Complex properties stretch the clock: a single-family rental with a clean closing statement moves faster than a $2,000,000 mixed-use building with 3 renovation phases. Document readiness is the variable you control, so having the settlement statement, depreciation schedule, and renovation invoices ready on day 1 matters.
Step 3: the tax work, roughly 1 to 2 weeks
Once the report lands, the asset detail has to become MACRS depreciation schedules and a Form 4562, and bonus depreciation under IRC section 168(k) gets applied to the qualifying 5, 7, and 15 year property. For a look-back on a property already on filed returns, the preparer also computes the section 481(a) catch-up and prepares Form 3115 under the automatic change procedures described in the Form 3115 instructions. Amadae handles this step: it ingests the report, builds the schedules, prepares the Form 3115 when one applies, and files the return.
A worked timeline
An investor closes on a $1,200,000 short-term rental on December 10 and places it in service December 27. She files an extension in March, signs the study engagement in February, completes a virtual inspection in early March, and receives the report 5 weeks later in mid-April. Her return, with the study built in, files comfortably in May, months ahead of the October extension deadline.
When the clock actually matters
The study does not need to finish by December 31. What must happen by year-end is placing the property in service, under the placed-in-service rules in IRS Publication 946. The study only has to exist before the return that uses it is filed. The real squeeze comes from engaging in late February for an unextended April deadline, or from engineering firm backlogs during filing season. A look-back study has no deadline pressure at all, since Form 3115 attaches to the current-year return whenever you file it.