A bitch may reach sexual maturity anytime from 6 to 12 months of age and this is indicated by the first oestrus cycle, more commonly referred to as "coming on heat" or "coming into season".From this initial season, bitches come on heat every six months (for most bitches), and each oestrus period lasts about 18 – 21 days, but this differs from dog to dog, and it can vary from 14 – 28 days.