It can take up to three months before the first batch of babies appear. It takes a bit of time for a new rat colony to get settled in and breed. The last breeding group took about a month and half before the first litter was born. Gestation is 19-24 days so mine took about a month before they did bred.
It takes 3-4 weeks to wean a rat from birth, about 16 weeks hs for females to reach full size. Males take longer to reach full growth and can be much bigger than the female (almost a full half pound). Males can breed a female within 3-4 weeks so should be separated as soon as they are fully weaned.
Not sure what you mean by 3.5 mice. A small fuzzy rat is equal in size/weight of a small adult mouse. A fuzzy rat with eyes open is equal to a large adult mouse.
There are pleny of websites out there that can give you more detail on the rate of growth in rats and mice. Some of the frozen rodent online stores also give weight and length comparisons as well... Below is a link to one sight I found that gives a good showing on the size and weight of mice. They used to do it for rats but I don't see it anymore. Rodentpro has images of rats showing the differences in sizes between age.
Mouse Sizes and Weights
