Female dogs typically reach sexual maturity around six months old and that’s when they have their first “heat” or estrus cycle. During this stage, there’s an increase in estrogen levels, then a sharp decrease, and then her ovaries release eggs. Their estrus cycles typically last three weeks but can range between two and four. Dogs typically go into heat on average every six months, but this varies especially in the beginning. It can take some dogs around 18 to 24 months to develop a regular cycle. Small dogs usually go into heat more often — as much as three to four times a year. Large breeds such as Great Danes and St. Bernard might only go into heat once a year. Dogs will continue to have cycles, but the length of time between estrus will increase as she gets older.

To summarize, female dogs typically have their first "heat" or estrus cycle around six months old, but it can vary depending on the breed and size of the dog. The estrus cycle typically lasts between two to four weeks, and dogs typically go into heat every six months, but this can vary.