Admission is granted for a period not exceeding three years and is renewable, so the practical question is sequencing rather than duration. He should apply once the duties, the start date and the employing entity have all stopped moving, and not before.
Apply to a schedule that has stopped changing
The three-year maximum sounds generous until you notice that the admission is tied to the employment described when it was granted. If the mill reshapes the role in the spring, the authorization is matched to a job that no longer exists. Wendell should treat the following as preconditions rather than details: a final duty description, a named employing entity, a confirmed start date and an agreed period.
As a Canadian citizen he needs no visa and may present himself with the documents at a land port of entry or at a preclearance facility, or his employer may file Form I-129 with USCIS and he presents the approval when he travels; a Mexican citizen in the same position would ordinarily use the visa process instead. Renewal is available and there is no fixed lifetime cap on this category, but each renewal is a fresh examination of the same two findings, so a role that drifts steadily toward purchasing will eventually be refused even though the first admission was granted. Two habits cause trouble in the interval before a start date.
One is beginning the work remotely from home for the American employer. The other is crossing as a visitor and doing something productive while there. Neither should be improvised.