"If you are on a vehicle or enter a station with a few minutes remaining on your transfer, you can continue to complete your journey and do not need to tap if the transfer expires while you are on that vehicle. For example, if you tap on a streetcar 1 hour and 55 minutes after your first tap, and you are still on the same streetcar 20 minutes later, your transfer is still valid. When TTC fare inspectors query your PRESTO card, they will be able to see that your transfer is still valid."
www.ttc.ca
EDIT: I rarely use TTC these days so I don't know what would happen in practice, but the published rules seem pretty simple: Tap where you're supposed to tap and if the fare was valid at that time then it counts.