how long will apple support iPhone 7?

Apple just released another update for the iPhone 6, so while we have no way to know what Apple is actually planning, a reasonable guess would be that you will see updates for the iPhone 7 for the next couple of years or so.

When updates for the iPhone 6 stop, that would be a clue that updates for the iPhone 7 and iOS 15 will go another year or so.

We do not know what Apple plans to do. Typically the current and one previous main iOS versions continue to get updates but on several occasions Apple has unexpectedly released security updates for older iOS versions. Given the age of an iPhone 7 running on iOS 15, iOS 15 would be the next in line to be dropped.