Work backwards from the date the employer needs someone in place, and allow time for the support letter to be drafted, reviewed and corrected, which is usually the slowest step. A request presented at a port of entry can be decided the same day, but that speed is misleading if the documents behind it took two months to assemble.
The paperwork calendar, not the border, sets the schedule
A same-day decision at a port of entry makes the process look fast and encourages people to leave preparation until the last fortnight. The realistic constraint is elsewhere. An employer's counsel has to understand an unfamiliar profession, a supervising professional has to confirm their own credentials, transcripts have to arrive from an institution that answers slowly, and a first draft of a support letter is usually wrong in ways that need a second and third pass. Where an employer files Form I-129 instead, processing time is a further variable and should be checked against current published times rather than assumed. Build in a margin for one round of corrections, and avoid committing to a resignation date or a lease before the position is actually authorized.
Put the credential question before the departure date. A first review should identify the exact profession, list every fact the support letter must prove, and flag documents that need replacement or explanation. That sequence leaves time for the employer to correct duties that are too generic. TN requires genuine prearranged employment and does not permit self-employment; a Canadian can request classification when entering or an employer can choose an I-129 filing. Hypothetical example: A seismic-data interpreter is scheduled to begin after a software rollout, but the employer's draft describes only client support. The sensible decision is whether the duties can truthfully be defined as the claimed profession before the rollout date drives the filing strategy.