The general rule i've used is 1lb fruit per 1gal beer. From there, you have to factor in 1) what type of beer it is, and 2) what type of fruit you're using.
If you've got a strong flavored beer, you'll need more fruit. If you're using a subtle fruit (cherries, blueberries, etc.) you'll need more fruit. If you've got a strong flavored beer AND a subtle fruit, you'll have to use more fruit than you think you should.