it depends on the age under 1 week=every 1-2 hours 1-2 weeks=2-3 hours 2+ weeks=the eyes open and they eat very 3-4 hours and get weined from their milk
It's best to let the mommy mouse take care of that, but if they're orphans or the mom abandons them. About every hour until it gets older, then you can do every 1-2 hours and then three hours then 4.
You can feed them kitten milk replacement, which is available at most pet shops. As they grow older you can try soaking bread in the milk and feeding them that until they can consume solids.