Female dogs typically go into heat about twice a year, approximately every six months

. However, this frequency can vary depending on factors such as breed, size, and individual differences. For example, smaller breeds may go into heat more frequently, sometimes up to three times a year, while larger breeds may have longer intervals and only cycle once a year

. The heat cycle usually lasts around 14 to 21 days (2 to 3 weeks), during which the dog is fertile and can become pregnant

. The cycle includes phases such as proestrus (beginning with swelling and discharge), estrus (when the dog is receptive to mating), and anestrus (a resting phase between cycles)

. Dogs generally have their first heat between 6 and 24 months of age, with smaller dogs tending to experience it earlier (around 6 months) and larger breeds later (up to 18-24 months)

. In summary:

• Frequency: About twice a year (every 6 months), but can range from once to three times yearly depending on breed and size.
• Duration: Approximately 14-21 days.
• First heat: Usually between 6 to 24 months of age, earlier in small breeds and later in large breeds.

This cycle continues throughout the dog's life unless she is spayed, which stops the heat cycles entirely