I breed mine a week or two after weaning (I wean kits at 4 to 5 weeks old). Just because I am nice to my does and like to keep them fat. And I have nine does, so keeping them all pregnant all the time would give me more juvenille rabbits than I have growout pens for!
The more heavily you breed your does, the shorter their lifespan will be. It makes sense if you think about it. You could get 5 or 6 litters a year out of a doe for two years, or get 3 or 4 litters a year out of her for 4 years. I get a little fond of my good mama's and like to have them around for awhile, but I'm not exactly making any money off of this rabbit business, either.