Admob best practice: how many times do I request, and how long should I show a banner ad?

You should set your banner ads up to refresh every 30-60 seconds. I would recommend that you display them continuously throughout your game, don't show and hide them.

For interstitial ads you should request an ad on start up and them display it at a significant break point in your app, such as before/after a new level. You then need to request another interstitial ad.