If you promise not to post any "man, that was real stupid" posts 'cause I already know it was, I'll give you a definitive answer. When I was young, a friend and I would shoot #8 shot at each other from standard vel. 12 ga. We were just about 150 yds from each other. (I just remeasured the distance and that is about right.) The shot would reach us, but it was like someone had taken a small number of #8 pellets and simply tossed them at us from a few yards away. In other words, they had no power left at all. So that would be the maximum for that particular shot size. Of course, smaller shot or higher vel. shot may go considerably farther.
One way to test the distance, if you want to, is to find a deserted calm lake that's long enough and take your best shot, so to speak. You should be able to get a good estimate of the distance by looking for the splash area. Of course, your local laws apply, i.e., you might need to use only steel shot for this experiment.
