Hi Dox.
I would say that 30 yards is the limit for ethical bowhunting, don't get me wrong, there are good shots that can shoot from longer ranges but, you as a bowhunter must know your limitations. As Andries said if you want to shoot at 20y practice at 40y and the same for 40y practice at 60y.
Even 20y is a problem for some bowhunters, they wound animals like Impala and Warthog then they blame it on the size of the animal they then go for bigger animals like Blue Wildebees wich have a bigger target area. This is absolute nonsence, it is simply a person that doesn't practice enough and is yust not ready to hunt with a bow yet.