I recommend breeding rabbits as soon as possible. For small breed rabbits, this would be about 6 months. For larger breeds, about 8 months. The longer you wait to breed maiden does, the more difficult it can be for them to conceive and kindle.
Some rabbits mature sooner than others. If you have a small breed rabbit that's on the larger/broody side, you may try breeding her as early as 5 months. Same with larger breed rabbits that mature quickly. I usually wait until that 6-8 month mark because I find that they're not only physically mature but also mentally more mature.