How long can i keep my number if i do not pay

I think you would be better off asking CS about this on the chat.
https://fizz.ca/en/faq/resolving-issues-step-2

Chizz :))

All I can tell you is you'll have no call in/out , and no sms out if you don't pay . It's effective immediately. If your target is keep your phone number, go with a company called Speak out, who will keep your phone number for $25 per year.