Is 6 hours usage and 7 hours standby good battery life for iPhone 6s?

It will depend largely on what the usage consisted of. If you were playing a heavy duty game, yes. Listening to music, no.

I'm getting following usage time on iPhone 6S

5:30 hr on 3G

6:30 hr on Wifi

Almost 95% use 3G or Wifi

A lot of factors influence the battery life of a phone.

The most prominent is network signal strength. In an area of poor coverage, the battery will drain faster.

What puzzled me is your standby time: only 7 hours of standby is a very poor result.

My usage time vary between 6 and 8 hrs, but the standby is more than 20 hours.