It depends on how you breed, like mentioned above..
I personally do harem breeding, with no breaks, 1.3 in a tub. My females start breeding at 4 months, and I retire them at 12 months.
I used to pull preggos, and put in individual tubs, but I find that harem is still the best way to do it.
I built a 72 slot condo for preggos, it wasn't cheap either... I've since refurbished it and gone back to harem breeding....
My females have 10-14 averages... I pull any that don't produce better than that after two litters.