21 days is what I read somewhere~ three weeks give or take seems to be the fact. No need to fuss over it~ three weeks.
It will take the couple a few weeks to get "friendly" so don't go start messing with them if they don't pop out pups in three weeks. Most of my colonies go 7 to 10 weeks from weaners put into the colony to producing pups for me. Takes a while to grow~ figure out the mechanics~ and then actually start working. The more you mess with them the longer it will take. Don't mess with the colony once you put it together. If your going to want more females do it now rather than later~ the colony rarely will allow a "new" mouse.
If I have a pup killer it is usually another female. Sometimes even the momma. If it is the male he will usually only do it to a couple litters~ once he has been with the girls long enough to be sure all the pups produced are his, he rarely continues killing them. Females that kill will usually keep it up until the other females kill her, beat her into submission, or you discover her and cull her.
Good luck!