I had Emma fixed at 7 months, before her heat, because of the contract with the breeder. Natalie came into heat around 9 months and I had her spayed after her heat, as I didn't get her until then. The breeder was planning on having her spayed around then any way.
It depends on the lines I think as to when they come into heat. Check with the breeder as to when her girls had their heats.