How long before iPhone 7 is obsolete .

Hardware obsolete? See this document -> Obtaining service for your Apple product after an expired warranty - Apple Support

As for iOS support, Apple has no published limits. It will continue to get iOS updates as long as the changes in iOS remain compatible with its hardware.

Jetster64 wrote:

How long before iPhone 7 is obsolete .

Nobody in these user to user technical forum would know the answer to that, you'll have to wait until Apple announces it.