If you want to extend your stay in Canada, you should apply at least 30 days before your status expires.
If you apply to extend your visitor visa expires while you are waiting for a decision, you still have temporary resident (visitor) status. This is known as implied status. That means the law implies you are a temporary resident. That status lasts until Citizenship and Immigration Canada (CIC) decides on your new permit application.
If you have a single-entry visa this allows you to enter Canada once. If you leave Canada during your authorized stay, you must get a new visitor visa to re-enter Canada, unless you visit the United States or St. Pierre and Miquelon.(You should have visas of these countries) If you visit those places, you can return to Canada without a new visa, as long as you:
return within the period that the Canadian immigration officer initially authorized when you first entered Canada (on your visa, it is either a handwritten date or 6 months from the date of the entry stamp); or
have a valid visitor record, work permit, study permit or temporary resident permit (authorizing re-entry), and return within the period that the officer initially authorized.
There is no statutory limit on the number of times a person can extend visitor status. Instead, the officer will consider the history of the applicant, the purpose of the visit, and whether there is a valid reason to continue visiting. The primary concern of the officer will be that the applicant intends to remain in Canada permanently, in which case the extension would not be granted. In some circumstances, numerous extensions can be granted as they make sense in the overall context. For example, where a parent seeks extensions while waiting for permanent residence. In other circumstances, there appears to be no valid reason to remain in Canada, and the officer will deny further extensions.