Generally, the cycle lasts 21 to 28 days. I've read 7 days in, 7 days standing, 7 days out. And then I've also seen it said that it's 10/10/10.
Here are a few links that explain it:
Reproduction, The Heat Cycle, Signs of Heat TalkToTheVet's info-Dogs Heat Cycle How Long Does the Heat Cycle Last in the Dog?