Approximately three weeks, although this can vary from dog to dog and even from season to season. Usually the first week they start to swell and then start losing blood, sometimes this can be hardly noticeable, sometime heavy and persistent, then this usually turns a lighter colour for a few days, followed by the bitch still being swollen but showing no colour at all. By the end of the second week they are usually ready to be mated and will do all they can to find one! This can last up to another week - 10 days so you have to be careful. Usually the swelling goes down and they are back to normal by the end of the third week, although it can go on for another week or so. I had a dog in kennels while my bitch was in season, he came back on day 28 and mated her!