Long enough to gather records, obtain a second opinion, and take tax advice, but short enough that documents stay current. There is no deadline pressure that justifies skipping those steps. Published descriptions of vetting in weeks describe one part of the process and are not a commitment about the whole route or any personal case.
Set a decision date of your own
Pick a date by which the household will either apply or stop considering it, and list what must be known by then: the eligibility opinion, the confirmed family list, the full budget, and the tax position. Urgency created by anyone else is a reason to slow down, not to hurry. If the required facts are not settled by that date, extend deliberately rather than drifting into a nonrefundable payment.
There is a reason to set the date rather than let the process set it. Documents age, family circumstances change, and an exploration that runs indefinitely tends to end either in a payment made under pressure or in nothing at all. A stated decision date with a list of what must be known by then gives the household something to work toward and a legitimate answer to anyone applying pressure.
If the required facts are not settled by that date, extend it deliberately and record why, so that the extension is a decision rather than a drift.