Mice usually won't over or under eat, you can put in a dish of food and let them eat as much as they want throughout the day. However, there are exceptions, some breeds of mice are succeptable to obesity, so if you have a fat mouse, you may want to trim down his portions and lay off the fatty foods. My mice eat out of a shot glass, and I usually fill it up once in the morning, once at night. However, they do pick through it, and spill some of the food during the course of the day. Make sure you remove as much old food as you can if it is not the dried variety as mold and fungus can start up in a cage in a short period of time. try to keep the food in a seperate area from the water to avoid getting the food wet, as it will stink up your cage and be a fuel for mold.

What can mice eat?

Mix these up together to give your mice a nice variety. You don't have to use them all, these are just suggestions. Also there are pre-made rodent mixes that are generally made for hamsters, mice can eat them, but they can be the fatty side. I feed my mice pre-made mix, and mix it up with a couple of cereal flakes. They don't seem to be fat at all...

laboratory pellets (very nutritous, but your mouse will need some variety to supplement this, as they don't find them extremely tasty)

cooked and cooled rice

cooked and cooled pasta

dried veggies

fruits (not citrus, can cause diarrhoea in some mice)

cooked and cooled beans

barley

oats

stale wheat bread

Treats

Give these to them in moderation and small portions

Cereals (my mice love Cheerios)

fresh bread

fresh veggies (lettuce cabbage and broccoli - not too much, can cause diarrhoea)

peanut butter

cat/dog food

sunflower seeds