German Shepherds usually have heat cycles occur every 4-5 months. Their cycles are generally shorter than many other breed of dogs.
A German Shepherd and many other large breeds may not have their first estrous or heat cycle until they are almost 2 years old. The cycles will continue until the dog is spayed. During the time of heat, a female's body is ready to become pregnant. The heat cycle repeats every 4-5 months usually but can vary based on the animal. Many people believe certain dates in the cycle have a larger chance of the dog becoming pregnant but this is not set in stone.