Your mouse should have access to commercial pellets or blocks all the time, so you don’t need to worry about portion control when it comes to this part of its diet. Let a mouse eat as much of this as it wants.
Clean, fresh water should also be provided at all times, as well as timothy or orchard grass hay.
Give your mouse about a tablespoon of greens per day. Avoid giving much more since an excess of greens can cause diarrhea.
Table scraps should only be given as occasional treats. Give a small piece of food such as cooked, unseasoned chicken, tuna or baked potato to your mouse a few times a week. You can also give it a small piece of fruit every few days.