Start when the duty description is being drafted, not when the start date is set. The slow parts are the employer's letter, a letter of good standing from the licensing body, and any state-side clarification about who signs the work.
The letter takes longer than the application
Hypothetical example: a Grand Bay-Westfield surveyor is given four weeks' notice for a corridor project that has already been awarded. The application itself is not what consumes the time. Getting a U.S.
employer's counsel to rewrite a duty paragraph so that it describes one listed profession, obtaining a current letter of good standing from the provincial association, and confirming how the state licensing rule affects his deliverables can each take a week or more, and they run in sequence because the letter depends on the licensing answer. TN admission is granted for a period of up to three years and can be renewed, so an eighteen-month engagement fits comfortably inside one grant. That makes the front end the only real constraint, and it argues for opening the file the day the scope of work is first discussed.