* Small breeds: Typically reach their first heat cycle between 6 and 12 months of age.
* Medium breeds: Usually go into heat between 12 and 18 months.
* Large breeds: Can be a little later, around 18 to 24 months old.
It's important to remember that these are just general guidelines. Some individual dogs may go into heat earlier or later than this. If you are unsure about your dog's heat cycle, it's best to consult your veterinarian.