The limits would be determined by the app users.
The usefulness of your app vs. the irritation of ads- including the timing, placement, type, repetition, and number of ads are all factors in the users' decision to use an app or not.
Also, keep in mind that if there are ad-free apps that are similar to yours...
well, word travels fast on the internet.
If your app is available on an appstore that allows for user to review apps, then pay attention to any that complain about there being too many ads, and do what you can to try to alleviate the issue.
Try not to have ads that interrupt gameplay, for instance.
Keep ads at 30 seconds or (preferably) below.
No more than one ad at a time.
That sort of thing.