I usually let the does tell me when they're done. Skye I had left for little over a year between her litter last summer and the one prior to that, and that baby didn't live. I was hesitant to try her again, but I did when someone had requested a couple babies out of her so I put her with a different buck and now, 8 months after the lost baby, she has a litter of 3 gorgeous babies that are all fat and healthy. She'll be five years old in April. She'll be retired end of this summer. I've also heard of a doe that's almost 8 years old(dutch) that's still eager to breed and torments males so as long as she stays healthy and has no problems her breeder has let her raise babies. She gets awesome babies out of her(I've got one of them in my barn) and as a 6 year old she rasied her own 6, and 2 dwarves at the same time. Wonderful doe.
So it does really depend on the doe on top of her condition.