The vets insisted I wait until they were 6 months old and one was not and I wanted them done together so I had to endure for 3 or 4 weeks iirc. The receptionist at the vets said it's a dominance thing and not a sexual thing. She didn't see the pink penis poking it's head out all the time though and cat booty going for it. I didn't know whether to get pink toys for them or what. So they were fixed, and I said so glad they will stop humping, it was getting very sex maniac styley and she said it will continue for a while yet. It stopped dead