The part of the Heat Cycle where you see blood lasts up to over a week. The actual beginning of the cycle starts when she begins to swell. Then you see blood, the blood thins, then it begins to change to a clear discharge, then she ovulates and there is no blood. This should be about the 10th to the 14th day of her cycle. This is the optimum time to breed or to keep her inside, depending on what your plans are. Once the blood has cleared, you still have a couple of days left where she might get pregnant. Most owners who do not want to breed keep their dam "safe" for about 3 weeks.
You can read more about the heat cycle here: http://www.dogbreedinfo.com/breedingheat.htm