There are four stages of the heat cycle.the first stage , proestrus, can last from 4-9 days. This is when you will notice a bloody discharge. the second stage Estrus, the bloody discharge will turn a pale yellow and this is the time the dog can become pregnant. this can last from 4-13 days. The third and forth stages diestrus and anestrus are basically inactive and can last from 50-80 days . During the third stage, diestrus, the dog can go through a false pregnancy.Here is more on the heat cycle. Keep in mind that each dog is different and the length of time for each cycle period may differ.