Well, the heat cycle is generally begun with the first notice of blood, but many people notice behavioral changes in their bitches prior to blood. The bleeding usually lasts for approximately three weeks. But somewhere, usually in the middle of the heat, the blood lightens, dogs go from interested to EXTREMELY INTERESTED, and she will often go from grumbling/baring her teeth at dog's advances to turning around, offering her butt and putting her tail aside for them (called "flagging" or "flagging her tail").
At that point for about 3 days she is in "standing heat." This is where she will literally "stand" for the male. And normally it coincides with ovulation, or rather the time when the egg will be ready for the sperm to fertilize it. She will be just as intent on getting to the boys as the boys are at getting to her, so, constant, never-ending vigilance!
After the 3 days, the boys will still try to impregnate her, but they will have less interest, and she will have less interest in them.
Heidi Ho, Odie
Joy-Joy, Bear Cub, Hepsi-Pepsi
Cujo2, Karma Chameleon
Ramona the Pest, Kojak -- who loves you baby?
Tiny Tinnie, Susie's Uzzi, Kaiah -- The Baby Monster.