Before the manager's last working day, if that is still possible. After the departure, drafting the description falls to someone who did not design the role, which takes longer and tends to produce a vaguer document.
Move while the knowledge is still in the building
The window between a resignation and a last day is the most productive time to gather duty evidence, because the person who knows the role is still reachable and usually willing to help. Ask for the reissued letter and the written role description in the same request. If the departure has already happened, ask the successor to work from the job requisition and team documentation rather than from memory.
The pattern a review exposes is a successor writing from a job posting that was edited for recruitment rather than for accuracy. The posting sold the role; it does not describe it. The record that ends the guesswork is the departing manager’s own written answer to three questions asked before the last day: what the position exists to do, who it reports to, and what the professional will produce in the first quarter.
Request it as a reply to an email rather than as a formal document, and it generally arrives the same week. Nothing shifts with the calendar on the legal point: the work must remain professional activity within the listed occupation, whoever describes it. Hypothetical example: an outgoing director of benefits administration answers those three questions in one message, and the successor drafts the letter from it two weeks later.