how long will iphone 7 be supported

iPhone 7 is not getting any further major iOS updates.

Ios 15 is the last version it will run.

The issues with no more major updates to iOS may not be as important as which apps are supported. I wouldn't anticipate that app support would stop that soon, but eventually things like banking apps stop getting supported when security updates to the operating system stop being issued. It wouldn't simply be that updates to the apps are no longer supported, but that the service (like connection to a service) stops support.