Most vets say 6 months in the UK and prefer to perform the op when they are not in heat although I don't think it's a huge problem if they are in heat. Our Milly and Ruby had their ops just before 6 months and I recall one of them was in heat. Ruby I think.
I certainly recall Milly being in heat from 5 months. What a tart! She wouldn't leave poor Fred alone
He didn't know where to look
Good job he was nutless