Long enough for the internal material to be gathered and approved, which is usually the slow step. Project timetables tend to be set before anyone considers the paperwork, and the paperwork depends on people whose priority is the project.
Start collecting while the project is still being planned
The material needed here is the same material a well-run implementation produces anyway, so the collection can begin as soon as the assignment is contemplated rather than waiting for a decision. Ask for the questions and the document list early, circulate them, and let the answers accumulate. Approval to use commercially sensitive documentation can itself take weeks in a group with several stakeholders, and that approval is easy to forget until it blocks everything. Set an internal deadline well ahead of the intended start, and treat a missed deadline as a signal to revisit the start date rather than to compress the preparation. The employer usually has more control over the start date than it believes it does, and saying so early is easier than saying it late.
Hypothetical example: Wen is needed to transfer a proprietary membrane-testing process before a customer acceptance milestone. An opening assessment sets the technical date beside the qualifying-employment date and the time required to collect internal proof. The project plan, training calendar, and personnel records settle which constraint controls. Begin with evidence that may require approvals from legal, security, or research teams; it is usually slower than drafting a letter. The predictable failure is announcing the move before confirming that the employee's knowledge can be documented without exposing confidential information. If access cannot be evidenced, a different staffing plan may be more realistic than a rushed narrative.