I don't know the exact number of days, but at a certain point, GPs are no longer obligated to invite subs when a skater w/d. Someone who knows the details could tell you here if Skate Canada is obligated or not at this point.
The "rule" is
It is mandatory that an Organizing ISU Member replaces any Pair who withdraws from the event with an alternate up to fourteen (14) days prior to the Team Leaders Meeting for that event.
For a Women or Men Skater, up to fourteen (14) days prior to the Team Leaders Meeting for that event, it is voluntary that an Organizing ISU Member replaces any Skater who withdraws from the event who would be the 11th or 12th entry but must replace with an alternate any Skater who withdraws if fewer than 10 entries remain.
For an Ice Dance couple, up to fourteen (14) days prior to the draw for that event, it is voluntary that a host country replaces any Ice Dance couple who withdraws from the event who would be the 9th or 10th entry but must replace any Dance couple who withdraws with an alternate couple if fewer than 8 entries remain.
At the fourteen-day mark, consideration will be given by the ISU Grand Prix Coordination Group to leaving a position vacant because of visa requirements or unduly high travel costs that would be incurred by the Organizing ISU Member.