How are frontend instance hours calculated on app engine?

Question

I have a simple online ordering application I have built. It probably handles 25 hours a week, most of those on Mondays and Tuesday.

Looking at the dashboard I see:

Answer 1

You're charged 15 minutes every time an instance is spins up.

If you have few requests, but they are spaced out, your instance would shut down, and you'll incur the 15 minute charge the next time the instance spins up.

You could easily rack up 4.5 instance hours with 18 HTTP requests.