Far enough ahead to have a firm start date, since admission is granted for a period not exceeding three years and is tied to the employment described. In practice the licensing and credential steps, not the immigration step, set the calendar.
Work backwards from the date the job actually begins
Hypothetical example: Sylvain runs the design side of an East Ferris design-build contractor and has been asked to join an American firm's healthcare studio in the spring. Architect is an Appendix 2 profession met by a baccalaureate degree or by state or provincial licensure. The immigration step itself is short: a Canadian citizen can present the evidence at a port of entry, or the firm can file Form I-129 and he can present the approval afterwards.
Everything slow sits upstream of it. Registration in the state where he will seal drawings, an evaluation if any of his education was obtained abroad, and a letter describing the studio's actual work all take longer than anyone plans for. Two habits prevent the usual failure.
Fix the start date before finalizing the letter, because a letter written for January and used in September describes a project that has moved. And request the period the firm can genuinely document, since an extension against real work is easier to obtain than an explanation of an authorization that outran the assignment.