Female dogs typically go into heat about twice a year, or every six months. However, the timing can vary depending on the dog's breed, size, and age. Small breeds may cycle more frequently, around three times a year, while very large breeds might only go into heat once a year. The heat cycle itself usually lasts between 2 to 4 weeks, with the most fertile period lasting about 5 to 13 days within that cycle.

Heat Cycle Frequency

• Most dogs have heat cycles about every six months (twice a year).
• Smaller dogs may cycle about three times a year.
• Large or giant breeds often have longer intervals and may come into heat only once a year.

Duration of Heat

• Heat lasts on average 14 to 21 days.
• The cycle has four stages: proestrus (4-20 days, beginning with blood discharge), estrus (5-13 days, fertile phase), diestrus (about 2 months, non-fertile), and anestrus (4-8 months resting phase).
• The estrus phase is when the dog is receptive to mating.

First Heat Timing

• Dogs can have their first heat anywhere from 4 months to 2 years old, often influenced by size. Smaller breeds tend to come into heat earlier, around 6 months, whereas larger breeds may start between 12 to 24 months.

These factors together define how often and how long a dog will be in heat during her reproductive years unless spayed.