How long after your hysterectomy should you wait to exercise?

Not long. You should begin walking right away, though you may not be able to walk as long as you did preoperatively. You can add light activities and gentle stretching as long as it doesn't cause pain or fatigue. Upper body exercises and biking are ok after a few weeks. Just don't do any abdominal exercises or heavy lifting for 6 weeks. Exercise is easier after a vaginal/robot/laparoscopy approach.
Talk to your surgeon. It depends on the manner in which the uterus was removed. Laparascopic, vaginal only approaches, and davinci robotic surgeries all have different recovery periods. So ask your surgeion.