How much are you supposed to feed a pet rat daily?

So I just got two pet rats the other day. They definitely do not like their rat food at all lol. We know that you can feed them other things like fresh fruit, vegetables, and whole grains. They love pea's! Its like a drug to them or something. They also love bran flakes! How often should we feed them these snacks? Are rats only supposed to have a certain amount of food daily?