Is there an art for the placement and frequency of adverts in a game/app?

I've played games where the game play has been ruined by a flood of adverts. It may look good on their figures, but only had the effect of me uninstalling them. I believe “exit” ads are taboo for some reason.

I try to balance my adverts so as not to swamp the app, and place them at positions that will not break up the flow of the user. One of my apps has a childrens' section, and the adverts in that section can be turned off via the options. Do you think it's fair to put adverts in an app or gamed aimed at younger children?

There must be a method that hints at placement and frequency!