Follow your vets advise. For every heat your female has puts her at risk for mammary gland tumors and Pyometra which can be fatal.
6 months old is appox, the time a females comes into first heat, some of the tiny ones it could be a month or 2 later.
Years back I had 2 females I did not have spayed, why? because at 6 months old they were so tiny, I didn't want the to go "under the knife", I was scared for them. When one was 13 years old she got Pyometra, her uterus was filled with puss and erupted causing puss to go though out her body. I rushed her to the vet where her underwent a full hysterectomy. At this time I had the world's worst vet that had no overnight care. I was given my dog back that was not fully out of anesthesia which really was not an issue, just a scary situation for me. For 9 days I watched my dog die before my very eye, every night I was rushing her to the vet, she wasn't eating, she had no control of her bladder. I took her to an emergency clinic because she was now walking into walls. They put her in an incubator and told me to call them in the morning. 7AM the next morning I called I was told 15 minutes after I left she had passed away.
I would never want anyone to go through what I did, or what my girl went through, the pain, the suffering.
6 months later my 10 year old showed signs of Pyometra, I rushed her to my now new vet, they performed surgery which was successful, they kept her for 3 days, she was doing good but died 6 months later.
I implore everyone that gets a female puppy to spay before first heat, do not breed, breeding should be left to the experts.