When I started my breeding it took a good 2-3 months before my first litter popped out, but after that they would bred really fast. Also, depending on how many rats you want, you could leave the male with the female and he will make her pregnant immediately after she gives birth. This does wear down the female and that's why I personally don't do it but if you need a lot of production you can do it just make sure to take out the previous litter before the mom gives birth again.