I am no good with this kind of math stuff, but I'll tell you that you can expect a female rat to give you anywhere from 10-15 babies every month if she is always kept with the male. A rat takes around 6 months to reach full size, but 'large' rats are sometimes not full grown. A full grown one is usually classified as 'jumbo'. I'd give males probably 4 months to reach 'large' status.