Brandon's right, 3 weeks is weaning time--there's no need to keep them with mom longer, she'll just force them to stop nursing quite soon. They're already eating solid food by then. If you really want your mom mouse to have a break, remove the babies at 3 weeks, she'll have a few days to recover from nursing before she goes into heat again.

I keep mine in colonies, though, it saves space, and I also keep the male in with them permanently, as I've had bad luck with my females killing or castrating unfamiliar males. I don't see much reason to keep females producing after about 4 litters, anyhow, so there's no time for them to develop any issues from being rebred immediately.