A 10lb bag would last you quite awhile if you started combine it with real foods in your diet. You should get a variety of protein sources, as they all contain different amino acid profiles and it's to good to get a combination of all of them.
Aim for a whey shake first thing in the morning, a post workout shake or pre/post workout whey shakes and a casein blend before bed. In addition also have a minimum of 3-4 whole meals a day would be my recommendation.