From the beginning. The maximum period is known on day one, and the alternatives that could follow it all take years. Deciding in the final year means choosing among whatever options remain rather than among the ones that would have been available with proper lead time.
The end date is knowable at the start
This is the rare situation where the deadline is visible years in advance and still catches people. The cumulative maximum is calculable from the outset once prior time is counted, and everything that could follow it has a long lead time. An immigrant petition involves a multi-stage process whose duration depends on the category and on visa availability.
A move to a different classification requires its own qualifying facts, which sometimes have to be built deliberately over a period. A return to Canada requires succession planning at the receiving company. None of those is a decision to make in the last six months.
The practical approach is to write the ceiling date on the corporate calendar next to the extension dates, and to hold a genuine review at the halfway point about what the company and the family each intend. Keeping the travel record contemporaneously also preserves whatever recapture may be available. Hypothetical example: a musical-instrument distributor wants its Canadian general manager in the United States before the spring trade-show season.
The first review creates a sequence: verify the qualifying relationship, document the completed foreign employment year, define the managerial role, and then choose a filing timetable. Payroll and travel records settle whether the year abroad is complete; the organisation chart and staffing plan settle whether the American role is truly managerial. Filing before these records exist can force a denial or a weakened case.
The business should set the launch date after the evidence calendar, rather than asking the evidence to fit an announced start date.