Nobody can put a reliable figure on that in advance. Vetting is described publicly in terms of weeks, which is a description rather than a commitment, and later stages depend on visa availability, any interview, and requests for further documents. Plan arrival windows that can move, and avoid signing leases or resigning against a fixed assumption.
Build the plan around movable dates
Choose reversible commitments while the case is pending: month-to-month housing where possible, school enrolment deadlines noted rather than assumed, and employment notice given only when a step is actually complete. Ask counsel which milestone would justify each irreversible decision. Keep a written record of what each family member is waiting for, so a delay affecting one person does not silently derail everyone's plans.
Review the schedule whenever an official instruction arrives. Add one further record to that written account of what each person is waiting for: the date each item was last checked. A staggered move produces a set of parallel positions that drift out of date quietly, because the person waiting has nothing to report and the person moving has enough to think about.
A single sheet showing, for each family member, what they are waiting for and when anyone last confirmed it turns silence into information. Hypothetical example: a household keeps that sheet and reviews it monthly, and the review catches a document approaching expiry for a family member still abroad, which nobody had reason to look at because that person's part of the plan had not yet begun.