Debatable..............4 weeks are better in general but no ill effects will arise from 21 day separations for most breeds of mice.
In my experience most not all of my mice stop feeding from mother on day 19 hence I see no reason to leave them longer especially bucks whom often turn to wimps when left with mummy and can then often take a while to get breeding satisfactorily.
Please note not all mice are ready at 21 days, judge your own breeding patterns but to be sure indeed leave them for the full 28 days.
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I have already been criticized for not presenting rubbish on the show bench so need to breed faster to obtain results so I can look like a real breeder to those that choose the easy path of breeding other breeders successful mice ( I'll be impressed if they still do well a few generations down ) also to those I do not dribble over, hence my findings may be biased.
( Last bit is a tongue in cheek dig at said critics, nobody here except maybe one ) :lol: