A Canadian citizen with a complete package can seek admission at a port of entry as soon as the letter and qualification evidence are ready. The I-129 route adds USCIS processing time. Neither should be attempted with an unfinished experience file.
The experience file, not the border, is the long pole
Hypothetical example: the distributor signs in early spring and wants her on site within three weeks. If she relies on a degree, the package can be assembled quickly: letter, degree, agreement, passport, and she presents it at a port of entry or preclearance. If she relies on five years of experience, gathering employment letters and engagement records from former clients may take longer than three weeks, and presenting a thin file to save time risks a refusal that follows her into later applications. Set the start date after the file is complete. If the distributor prefers the I-129 route for predictability, its processing time and any premium-processing option govern, and she should not begin work on a visitor entry while it is pending. Plan the ten months with a defined end, since extensions require a new letter describing continued or new scope, and each admission is judged on the facts of that day.
Hypothetical example: an urban planner has a project launch date but is waiting for a credential evaluation that the employer letter promises to attach. The evaluation, not the construction schedule, decides when a defensible file exists. Set a review date after every supporting record is in hand, then choose a port-of-entry request or an I-129 filing based on the employer's tolerance for timing risk. Do not fill the gap by having the planner begin paid U.S. work for her own consultancy: TN does not permit self-employment.