we had a couple of young field mice rescued from a neighbours cat last year, kept them in a tank for a few weeks, as they had minor injuries, then put the tank outside half buried in soil, and continued to feed them, they stayed around for a few days, then not seen again, so either they made it or didn't, but i had to save them, i'm too fluffy not to, and it depends what type of mice they are surely?? as to whether you can release them, i would just go for it, but make sure it has somewhere to come back to so it can get food and shelter, after 6 months, though it may be a bit reliant on you feeding it to be released and not survive