Do to recent changes in the Canada Shipping Act, 2001, pleasure craft licences are now valid for 10 years. (Updated)
You are required to renew your licence every ten years, but only if you have obtained a new license with an expiry date on it. Old licences with no expiry dates on them do not expire.
However, if any of the information on the licence changes (ex: change of name or address), you will need to update your licence and will then be issued a new licence with an expiry date.
See the Pleasure Craft Licence page.
Thanks to Greg Dargavel for obtaining the clarification from Transport Canada Marine Safety about the non-expiration on older licenses (second paragraph).