A Canadian citizen can apply at a port of entry the day the file is complete and, if admitted, begin work immediately, so the timeline is set by how quickly the supervising professional's letter and credentials are assembled. An I-129 filing takes longer but gives an approval before travel. Do not book a start date before the letter is final.
Let the letter set the calendar
The sequence that works for a technologist is: obtain the engineer's description of the support relationship, draft the employer letter around it, assemble credentials, then choose the route. A port-of-entry application suits an applicant whose file is clean and who can tolerate the possibility of being refused at the booth and sent home to correct something; a repair facility that has a contract start date tied to the inspection program may prefer an I-129 approval it can show to its customer. Either way, resist the employer's request to arrive early and 'shadow' the team before the application is made; unpaid observation of the work the applicant is about to do is not visitor activity.
Once admitted, the I-94 sets the period, ordinarily up to three years, and extensions can be requested through USCIS or by a fresh application at the border. Track the I-94 date, not the offer letter's end date.