After the qualifying year is complete and documented, and before the manager stops performing Canadian duties. Filing early with an incomplete year, or after the manager has effectively moved, both create problems.
File when the year is banked and the handover is planned, not finished
Hypothetical example: The first constraint is the qualifying year: a Portugal Cove-St. Philip's manager whose continuous twelve months completes in March cannot be the subject of a petition filed in February. The second constraint is that the manager should still be employed by the Canadian entity when the petition is filed, so the timeline should not have the manager resign or move to the U.S. payroll first. The third is that a Canadian citizen may present an approved L petition at a port of entry, and blanket petitions and individual petitions have different mechanics; the sequence should be petition, approval, then admission, then U.S. payroll. Premium processing shortens USCIS adjudication but does not shorten the year abroad. The company should plan the Canadian handover in parallel so that a named successor is ready, because the petition is stronger when it shows the Canadian operation will continue after the manager leaves.
Hypothetical example: a soil-testing firm expects its Canadian division manager's qualifying year to finish in six weeks, but the U.S. entity wants the manager on site for a product launch sooner. The company must choose whether to move the launch, use another authorized worker, or delay the petition; it cannot treat an almost-complete year as complete. A payroll calendar establishes the eligibility date, and a handover plan establishes whether foreign operations continue. Setting those two records beside the business schedule turns a vague urgency problem into a decision with dates.