Once registered, a trademark is valid for 15 years. Near the end of that period, the Canadian Intellectual Property Office (CIPO), i.e the trademark office, will send a notice that it is time to renew your Canadian trademark.
Simply respond to that letter and pay the renewal fee. The fee is currently $450, for renewal of registered trademark in Canada, but this fee will probably have changed by the time any new trademarks come up for renewal.
If you do not renew the mark, or if you do not renew it in time, it will be expunged. It would still be listed in the trademark database, but would no longer provide any protection. This also means that someone else could register the trademark or you could register it again yourself with a fresh application.