A dog can typically be safely spayed between 6 to 9 months of age for small to medium breeds. For large or giant breeds, it is often recommended to wait until they are closer to 12 to 18 months old to allow for more mature growth and development. Spaying before the first heat cycle is generally safe and even reduces the risk of certain cancers, but timing should be tailored to the individual dog's breed, size, health, and lifestyle. It is also advised not to spay a dog during her heat cycle due to increased surgical risks. Consulting with a veterinarian for personalized advice is important for the best outcome.