From the PCM FAQ:

5. What happens if my airtime expires?
If your airtime expires, you have 45 days to refill your account before your service is disconnected. If that happens, you may also lose your Petro-Canada Mobility phone number. Fortunately, you can reconnect your Petro-Canada Mobility service, but additional charges will apply and a new mobile number may be assigned to you.

Translation: You may be able to continue to get free services for 45 days after your airtime expires but then you may find yourself completely disconnected and lose your phone number.

As Clint Eastwood once put it, "You've got to ask yourself one question: 'Do I feel lucky?' Well, do ya?"

Well if the answer isn't on the President's Choice Mobile website then you're more likely to get an authoritative answer if you call them and ask. Most of the people here are users of SpeakOut and Petro-Can Mobile so it's unlikely they'd know.