Once a day would probably be heaps, and you will more than likely be finding left overs that will need removing unless you happen to have snails in the tank as well. Don't forget you can also give your shrimp all sorts of leaves to munch on Mulberry leaves are popular as are Banana Leaves and Indian Almond. Fruit wise you can give them banana's and almost anything else that you would normally give to fish so things like carrot, zuchinni, pumpkin, peas will all be nibbled by shrimp. I am betting even the fleshy skin from a mango or the seed (after you have eaten the flesh you can get off it) would also go down a treat with shrimp.