How long will Apple support an iMac?

If you are referring to updates on your Mac,If you’re referring to updates for your Mac, typically, you’ll receive 5 to 7 years of main macOS updates, sometimes longer. You will also receive come and go security fixes.

If you are referring to service after expired warranty, learn about your options for getting service and parts for Apple devices that are past their warranty period.

Obtaining service for your Apple product after an expired warranty - Apple Support

This should give you an idea: Obtaining service for your Apple product after an expired warranty - Apple Support

Without specifics about the model yearbiMac you are inquiring about all we can tell you is what Apple has done in the past.