I got Chaos from Animal Control. He was there because his owners decided to breed their purebred siamese so they could have some more kittens. They wanted two kittens, momma cat had 3. Chaos was dumped at Animal Control at 6 weeks of age, about half the age he should have been when he left his mother. If I knew who his previous owners were i'd give them an earfull. This isn't somthing to trifle with, this is LIFE.
I understand wanting to do what you want to do and doing it no matter what anyone says, but before you decide that, take a look at the thread entitled "Why do my kittens keep dying?" Not only did all of the kittens die, the mother cat died too. To breed properly you should have at least a four generation pedigree on both cats, know that both cats and their parents have been fully tested against hcm, immune disorders etc. You should have a Vet that understands your situation and get regular checkups on the mother as well as the kittens. The mother and father should be Vet checked before they 'go at it' so to speak. The kittens will need all their shots up to 12 weeks when its safe to let them leave their mother. None of this is cheap. One Vet visit alone for me is over $70 per cat. It's more when they need shots, up to $140
As far as your original question is concerned, it's all a matter of perfect timing. See, if a female cat is bred too soon then she won't have even finished developing yet. But if you wait too long she can contract Pyometra, a uterine infection that can be deadly and effects unspayed cats. Even experienced breeders have had problems with Pyometra.
If after all that you are still going to go ahead and breed, well all I can say is good luck to your cat and the kittens.