The timing of a bitch's first season can vary quite a lot within lines. For the most part, you're looking between 6-8 months. But the best way to find out about your specific bitch is to ask the breeder when her dam first came into season. I've found (anecdotally) that there is a hereditary aspect to this specific subject.
Agree with Ludi, this has been my experience also, with bitches in general.