Gestation period for a mouse is between 19 - 21 days, sometimes longer if you are breeding back to back. I have had female mice pregnant for 30 days with no problems, but this is rare.
The amount of time it takes for "labour" depends on whether this is her first litter or not. If this is her first, it could take several hours, if she has had a few litters already it may take around an hour.
Also... are you certain SHE is the one killing the babies? Or could this be from another female? If it is the mother, she could be lacking protein and adding a good protein source, like egg or some cooked meat. If she isn't getting enough protein in her diet, she will eat the young to fill that need. If you disturb her too much, she may feel threatened and kill the young.
When the female starts to give birth, it is best to leave her alone. Generally, with the exception of checking food and water, I leave the females alone for the first day after birthing. If you are in need of pinkies, wait at least a few hours after she has finished, then take what you need.
Take care and good luck...
Annie B. <:3 )~~