How to know when ur pinky rat is full?

ok so u people might not like it but i got a pinky rat from a pet store to try and raise. yes i know that it might have health probs. but i dont know when he is full. yes its a boy i check online to make sure. i mean i fed him the stuff they said to feed him. but i dont know when he is full hes drinking it and he looks a little plumper then when i got him. but im not sure the site didnt say how to tell when hes finished drinking