Hi Judith,
Good work so far ... raising an orphan mouse is quite a commitment to undertake.
If its eyes are not yet open then it is likely less than 12 days old, so should be fed every hour during daylight hours and every 2 hours overnight.
In addition to the milk replacer, you should also feed colostrum at least until they are weaned (at a ratio of one part colostrum to 3 parts milk replacer) ... your veterinarian or local wildlife rehabilitator should be able to help you resource that.
Mice should begin to wean around 9 days old, before the eyes are open, so you should try it with some moistened rusk, or breakfast cereal, or a small piece of apple, pear or banana too ... the sooner you can get it eating solids the better.
It would be unwise to release a young mouse now before spring, and you need to be aware that in raising an orphan of this age on its own, it is more than likely going to become imprinted on humans and so may not be suitable for release at all (not being equipped to bond with its own kind) ... so realistically, you may need to consider keeping it for the duration of its short lifetime ... in captivity it should survive between 18-24 months ... in the wild, average lifespan is probably closer to 2 months.
Good luck.