Well before the current authorized period ends, and independently of when the contract renews. The contract's renewal date and the immigration period are different clocks, and assuming they move together is a common way to run short.
Track two dates, not one
Put the end of the authorized period and the contract renewal date side by side, and work to whichever comes first. Ask the employer to confirm renewal early enough to support the immigration step rather than at the last moment the notice clause permits. If the employer cannot confirm in time, that uncertainty is itself something to raise with counsel before the period lapses.
Work backwards from the earlier of the two dates, and leave room for the employer’s internal approval, which is often the slow step. A renewal confirmation usually needs a signature from someone who is not the day-to-day manager, and finance may want the budget line reconfirmed before that signature is given. Hypothetical example: a histotechnologist’s engagement renews on 1 April while the authorized period ends on 15 March, so the immigration step has to be ready in February, before the contract’s own renewal machinery has produced anything at all.
Ask for a written confirmation of intent that predates the formal renewal. Bear in mind that a request of this kind turns on whether the duties fit a profession that appears on the USMCA list and on meeting that profession’s qualification requirement, whichever forum is used. If the employer cannot confirm in time, record that plainly and raise it before the period lapses rather than afterwards.