A Canadian citizen with a complete file can be admitted at the port of entry on the day of travel, so the schedule is driven by document assembly, not by a queue. If the employer prefers a USCIS petition, allow for the current processing time or premium processing.
The trial calendar sets the deadline, the file sets the speed
Hypothetical example: a Haldimand County plant breeder with a crop-science baccalaureate accepts a role designing field trials for a U.S. seed company that needs her in place before planting. Plant Breeder is a listed profession with a baccalaureate rule, so the question is sequencing.
Work backwards from the first field day: the employer letter must be signed, the degree and transcript in hand, citizenship proof ready, and any research-station housing arranged before she drives to the crossing. If the household wants a decision in writing before she resigns from her Ontario employer, the employer can file Form I-129 and pay premium processing, which converts uncertainty into a dated answer. The initial TN period can be granted for up to three years, so the offer should state a duration that matches the trial program.
Do not let the employer's urgency compress the state of the file; an incomplete letter refused at the port costs more time than a week of preparation.