As far as I know, 6 months is the standard. Some shelters spay the little ones way too early as far as I am concerned, but their concern is that they do not want cats reproducing under "their watchful eye"....so they often do it at 3 or 4 months. The same is true with male kitties. All that said, I think you will be fine with 6 months, keeping the little one inside and your doors secured so that if she does become "receptive", no unaltered male cat can come courting!

Juni Rose is scheduled for September for her spaying. She is so, so tiny now....I may just wait another month. My vet really encourages spaying and neutering.....at six months.

Good luck. She will be fine!