A cautionary tale, if I may...
I purchased a 1.2 mouse colony, and set them up as pets would be set up...in a tank with a running wheel, a nice hide, fancy substrate and food "stuffs", a rope to climb and walk on...all the things a happy pet mouse could want. My daughter, who is almost 3, LOVED "her" mice, and wanted to play with them. I let her...everyday, but not "out of control" lengths of time.
Fast forward to 3 months later...no babies. No mating. In fact, the male, whose testicles were swollen at the time of purchase, indicating sexual maturity and preparedness, were no longer swollen, and visually were almost identical to a female's genitalia.
So I did some research, and discovered that mice can actually be very sensitive to external stimulation that wil inhibit breeding. Too much handling, interrupted day/night light cycles, odd sounds and disturbances...all of these things can cause mice to lose interest in breeding.
So I went to the store and bought a new 1.3 colony, put them in a different aquarium, and placed them on the top shelf of a closet, not to be disturbed other than cleanings, feedings and occasionally checking on them. They were mating the second night I had them. I have had the new colony for less than 3 weeks, and I have 2 that "look" pregnant, in that the bellies are visibly swollen compared to the rest of the body size. I removed one of the females from the first colony, put her in with the second colony, and she, too, appears to be pregnant, in less than a week from the introduction to the "Do Not Disturb" colony.
The second colony has only a home, a food dish, some cholla cactus stem chew toys, and a water bottle. There is no "excercise equipment", they do not get handled, their location in the closet prevents my bedroom light from interfereing with the natural light cycles from the window that they recieve, there food is pellets, mixed with the occasional dry fruit and seed, and their bedding is pellets with a smattering of cotton to soften it inside the hide.
The bottom line...decide if your mice are pets or breeders, and stick to the plan. I found out the hard way that pets are not good breeders as far as mice are concerned...