I usually have a pretty set routine. GSD's can pretty much go forever if they want, so it's hard to wait for a point where they decide they are tired. Plus, purely physical exercise will not be enough by itself to tire your dog out in a balanced manner, it needs to be interspersed with training.
For example, my dog normally has a one mile walk in the morning and a two mile walk in the evening. Each walk is followed by a training session, and at least a couple times throughout the day we play fetch or tug with training mixed in. On days when I'm free, we'll do a lot more because I have time - usually an 8-10 mile hike. But we still use that time to practice trail manners, obedience, and recall.
More energetic dogs may need a bit more, but it's a myth that you have to walk a GSD 20 miles a day to keep them exercised. In fact, if that is the only stimulation they are getting it could even be frustrating because they are getting a lopsided workout. Again, you need to work the body and brain to get a truly tired GSD. Plus, you don't want to have her constantly dictating how much is enough time walking or playing with you. As long as you are providing adequate time for exercise and training, your dog needs to learn to be able to settle between opportunities for physical and mental stimulation. Oftentimes, mine will get a walk then a training session, then a food toy like a Kong to help get him into the routine of physical then mental exercise (or both in the case of tug or fetch!) followed by settling in the house.