i normaly watch them and see if there eating but the best way to know when to take them out is when you see them drinking from the water bottle or what ever it is your useing.mainly if there drinking water then you can seperate them from mom.i breed asf rats and mice and i been doing that for a long time now with no problem. if you have runts or any that are realy small leave them in with mom for a extra week or 2 so they can faten up alittle.hope that helps