Uhm for me, average litter size is about 6-10. And i believe the litter sizes are based on the females genetics. I had two females that would have 16 every litter, whereas now all of mine on average have around 8ish. Having four is rather small, but her next litter will probably be bigger, but I think its more of just luck than anything. Could be genetics. Ive never rested my males, they do that by themselves. My males tend to breed more efficiently around 6 months of age and thats when I start to keep new males from my litters. Once you've had a few generations, I think that's when you should start outbreeding, that way you can see what you need to work on. I started with feeder breeders, and gradually bred them by waiting and seeing which ones were all together healthier and resistant to respiratory illness and lived the longest. At first they would only lived around 6 months. Then after about 2 years of keeping the healthiest, I could finally start outbreeding for other traits like colors, hair textures and ear shapes, temperments, etc. I usually wait until the females third litter before I decide if she's worthy of holding up the line because by that time she's about a year old. That way I know her temperment and how healthy she is. By now, I never have health issues, and it only took me two years to breed away from that. My males have successfully bred until around 2 years of age nonstop. They just slow down a bit after about a year and a half. But now I usually start selling them back to the pet store for store credit once they hit a year since I dont need to worry about disease anymore. I have around 25 females and about 5 breeding males. Probably buy a new rat or two every year to outbreed with. Mostly just to add a new color or pattern. I'm working on blazes, odd-eyes, and rex right now. Though I also have blue, mink, siamese, agouti and black based, dumbo, standard ear, and hairless that I work with. All from outbreeding. My line started with only black based blues, and pink eyed rats, and now after 6 years I have a lot of variations to work with, so if you find your rats are carrying hidden genes, youll only be able to find out by keeping your line or getting lucky with outbreeding. I'd say just buy your first set of rats to work with now, and wait and see what you need to work on. Just remember to rest your females, because they will get worn out. I usually give them around 2 or 3 weeks.