With mice I always wait till they're twelve weeks or older to breed. I don't weigh them but any that look thin don't get bred.

Weaning occurs at 3-4weeks of age. I tend to wean at 3 weeks because I like my breeders and I can tell when mom needs a break. If moms looking super healthy still I'll leave them together a little longer cause the longer with mom the faster they get big it seems.

Make sure you separate at 4weeks or you'll get incest.