
I think it's breed dependant and personal.
I personally have only bred from one bitch who had reached 7, and that was only a month, but she was nearly four having her first litter, and the earliest a few days after she reached two.
Ideally I like to have litters at 2 1/2, 4ish, and 6, then spay after the last. My breed is medium size (approx. 20kg adult bitch).