How long is a bitch in season?

A female dog is typically in season for about 21 days and consists of three phases.

The first phase, proestrus, is when the bloody discharge occurs. Proestrus typically lasts 7-10 days and is noted for being a period when male dogs are strongly attracted, but the female wants no part of them.

The second phase, estrus, is when the female ovulates and can become pregnant. It usually lasts 4 to 7 days and is characterized by a change in the discharge from red to pink and finally to clear. Ovulation typically occurs between the eighth and tenth day after bleeding first starts.

The third and final phase, diestrus, typically lasts 7 to 10 days. During this period she still smells attractive to males, but will again want no part of them.

Females should be spayed at least one month before their next heat, or one month after their last. Spaying decreases her risk of several types of cancer, including mammary cancer, in addition to preventing unwanted pregnancy.