Hi Christian.
Advertisers are not very fond of their ads being wasted. This is one of the reasons why no one does CPM (Cost Per Mille or per 1000 ads shown) campaigns any more. In the past, people set fast rotation times on ads to get the ad count up. This creates a poor user experience, slows down the network and eats through the advertisers inventory in a hurry. Advertisers realized they were not getting value from this.
Today most ads are a CPC (Cost Per Click) type campaign or CPA (Cost per acquisition or sometimes called CPI - Cost per Install). You get paid for clicks (or installs, conversion, getting new users). If you’re cycling ads every 20 seconds you’re not giving your user time to click on ads anyway. Banner ads during points of player action will never get clicked and you’re just burning through the ads and it’s not helping anyone: You, the advertiser or the player.
While I don’t know that you will find many vendors who state a limit, those that do may give you options of 30-60 seconds for auto rotation and this should be a good guideline of what’s acceptable. Put your banners in a part of your app that the user isn’t under pressure to interact with the app (i.e. time sensitive game play). A game like Angry Birds, you have plenty of time to interact with an ad, a tower defense game? Not so much. Leave them up long enough for the user to have an opportunity to see and interact with the ad.
Rob