When I was younger and dumber, some friends and I decided to ride down to the beach. The only problems was, we didn't live anywhere near the beach. None of us had special training or bikes, we weren't enthusiasts or serious riders, we just hopped on whatever we had and started riding. The 50 miles going to the beach was pretty easy, mostly level or slightly downhill, with the winds at our backs. Coming back nearly killed us, agaist the wind with a very slight, but noticable elevation gain. We made it to within a couple miles of our starting point and just collapsed on the side of the road.

So I'd say it's highly dependant on terrain. For an average person, 50 miles can be reasonably accomplished on flat ground, but add in some hills or wind and people will start to have trouble after a few miles.