I just had this problem. It was a puppy and wouldn't poop even after 30 minutes. I waited over an hour one day, but still no poops. Since I am being paid for only 30 minutes I felt that this was too excessive on my part. I let them know every time that they didn't poop. It was also in the middle of a heat wave and did not want to keep the dogs out in the heat for too long. I've come to the conclusion that 40 minutes is tops for waiting. The puppy, which was an 8 week old goldendoodle could very well be growing out of a mid day poop, so I wasn't going to stress about it anymore.