How long is a long time? How many groups? If you have more than one group of mice, switch males. Or take the entire breeding groups and switch cages without cleaning them... the "other male" scent will get the male's hormones ripping. How large a cage do you have them in? how old are they? I can't say anything about rats, just mice. What temps do you house them at? what's their substrate.
The first mice I ever got were December 22nd, and their first litters were February 1st. 6 weeks.