How much hay should you put in your guinea-pig cage?

Guinea pigs should always have hay in their cage. Feed them as much as they want. If you want to spice up their forage diet a bit, getting grass from outside (not near a roadway, sprayed with pesticides, or a dog's "bathroom" :). If your guinea pig is getting fat, I don't think it is because of the hay. Read up on what to do then, and if you want a second opinion ask your vet.