Answer:
Mice are able to reproduce at 4-5 weeks of age.
It's a 3 week gestation period for female mice, & they can become pregnant straight after giving birth.
Now, as mice live for about 2 years, if a female mouse gave birth every 3 weeks, they could have roughly 35 litters in their life time, each litter from 1 to 18 babies.
That's the females, the males can breed every hour.