I have a valid visitor visa for 8 years.
"Sigh"... you have a TRV that allows you to travel to Canada and be assessed by CBSA for entry into the county that is valid for 8 years. This doesn't mean you get to stay for 8 years straight. Your visa is actually the stamp they put in your passport at immigration. If it has no date, you can remain 6 months. If it has a date, you have to leave by then. You can apply for an extension 30 days before it expires.