Female dogs typically go into heat for the first time around 6 months of age, although this can vary widely depending on breed and individual factors. Smaller breeds may start as early as 4 to 6 months, while larger breeds might not begin their heat cycles until 12 to 24 months old. The average across all breeds is usually around 9 months. A female dog's heat cycle, also called estrus, occurs roughly twice a year (every 6 to 7 months), but this can vary. Each heat cycle lasts about 2 to 4 weeks, with the dog being most fertile around day 10 of the cycle. During this time, physical signs such as swelling of the vulva, bleeding or discharge, and behavioral changes like increased vocalization or interest in male dogs are common. It is generally advised that dogs should not be bred during their first two heat cycles as they are not fully mature yet. Spayed female dogs do not go into heat. In summary:

• First heat: Usually around 6 months, but can range from 4 months to 24 months depending on breed and size.
• Frequency: About every 6 months (twice a year) on average.
• Duration: Lasts 2 to 4 weeks.
• Signs: Vulva swelling, bleeding/discharge, behavioral changes.

These details provide a typical timeline and what to expect during a female dog's heat cycle.