An infestation might be thought of as requiring hundreds of mice, but even one or two mice in your home can lead to an infestation if not addressed. Female mice have a gestation period of 19 to 21 days and can give birth 5 to 10 times a year, with each litter ranging from 3 to 14 pups. They can begin reproducing at just 6 weeks old. Mice residing indoors have an average lifespan of 2-3 years, making it crucial to address even a small number of mice promptly to prevent a full-blown infestation.