It varies. On average once a year, but if your dog has its first season at say 10 months of age, it usually follows that it will have a season every 10 months. Some b!tches dont have a season until they are over 12 months old, and some have their first season much earlier. My friend had a female that had one season at 14 months, and never had another one in her life.
Can be every six months, but some dogs can be yearly, a lot of greyhound bitches are yearly. As jules says not only can it vary from dog to dog, but it can vary from season to season, so if she has one the next may be after six, seven, eight or more months, although in my experience the majority are every 7 or 8 months.