I leave Monday on the 10 day Alaska trip out of San Francisco and over the last 3 days I've received at least 2 phone calls, 3 emails and 2 text messages all from Carnival alerting me about the ArriveCAN requirement. So that seems to be the timeframe, you're not going to be notified by Carnival until close to your cruise (less than a week), which makes sense to me because Canada's requirements continue to change and that's out of Carnival's control.