Most of this is very arbitrary, but I guess the answer to your question, for me, would be that if you aren't confident in making the shot(and that the animal will likely still be in the same spot), then it is too far.
That is going to vary with every situation, depending from how I feel that day, the disposition of the animal, and which animal I am trying to kill. Some animals very rarely will move when the arrow is in the air. I water buffalo is a good example of this. I have heard that elk are much less likely to, as well, although I am not sure if that is assuming a further average shot distance.
I would say that 80% of the arrows that I shoot in practice are from 30 yards away, with maybe 5-10% from 40-50 yards away, and 10-15% closer than 30 yards. As elk season is getting closer, the percentage from 40-50 yards is going up. I plan on being very comfortable shooting from that distance before my trip, and I really don't care what anyone thinks about it, if I have a calm animal between 35-40 yds away, I will most likely be taking the shot if I don't think that I can get a closer one.