Plan around the admission period actually granted rather than around the length of the project, and plan the extension before you need it. TN admission is granted for a defined period, extensions are possible, and each one is assessed afresh against the same occupational and qualification tests.
The project schedule is not the immigration schedule
Confusion usually starts with an offer letter that recites a multi-year engagement, which reads to the applicant like a guarantee of a matching admission. It is not. Hypothetical example: an urban planner from Saint-Cyrille-de-Wendover is retained on a forty-month municipal master-plan project in upstate New York, and the offer letter states five years; at the counter she is admitted for a shorter defined period, and her employer has made no arrangements for what happens when it ends.
The workable approach is to diarise the expiry the day it is granted, decide months ahead whether the extension will be sought from inside the United States or by a fresh application at a port of entry, and confirm the employer will still describe the same listed occupation at that point. Two changes matter more than the calendar. If the duties drift away from the occupation named in the original file, the extension is a new argument, not a renewal.
And if the role becomes permanent in substance, TN is the wrong instrument, because it is a temporary category and its temporary character is examined each time.