Begin as soon as the employer's intent is in writing. A port-of-entry application can be decided the same day, but assembling a licence confirmation, translations and a correct letter typically takes several weeks, and an I-129 petition adds USCIS processing time.
Work backwards from the evidence, not the calendar
The slowest items are rarely the government steps; they are the documents other people control. A professional order may take weeks to issue a confirmation of standing, a university transcript request has its own queue, and a U.S. employer's legal department may revise the letter several times.
List each item, who produces it, and a realistic date, then add the government step at the end. For a port-of-entry application, plan to apply on a weekday with time to spare and not on the morning the lease begins, because a refusal or a request for more evidence needs room to be handled. For an I-129, check current USCIS processing times and decide whether premium processing is worth its fee.
Do not resign from a position in Québec until the status is granted; an offer and a planned date are not admission.