This is what my daily diet looks like for my two (female) rats ("rats" for scale). Their main food is Oxbow's Regal Rat, and because they like to forage and whatnot I also put Tropical Carnival in their bowl (can be a grain/seed mix you find in your local pet store). Most people suggest not to use grain/seed mixes as the "main" food because the ratties pick and choose what they eat. The scoops in the picture are exactly what I put in their bowl every night (each scoop is 1.25fl oz, they are actually travel toiletries containers I got from walmart, i think). It keeps both of them fed and it's enough for them to randomly nibble at throughout the next day. Sometimes I'll empty out the bowl to get rid of the unwanted seeds, etc.
That is what they get 24/7. Whenever I can I get greens from the kitchen for them, and very occasionally a nugget of meat. But my go-to treats are dog biscuits. Right now I'm loving these "gingerbread men" because my rats look evil when they eat them >:D.
Good luck! Let me know if any of this was confusing.
edit: As to how often I treat them, I limit to no more than 5 biscuits per rat per day.