Another one of those totally "dependant" questions....
It all depends on where I hunt and what I'm hunting for. The three big bear stands that we build in the trees were 20'-25' because we wanted to be up above them so small movements in the stand didn't give us away. I will usually go about the same when I hunt from my climber. Again I want to be out of their line of sight. Too high and you don't have a good double lung shot. I might not go so far up if the tree has a lot of low branches. I like to use the surrounding branches for cover as well. I have roped them and pulled them close for additional natural cover. It also depends on the terrain around me. I hunt in the mountains so I might have to go higher to get a good angle for a shot. I don't like to go too high because the tree gets smaller and the stand starts to lean forward. I didn't set my stand up correct one time and it leaned forward. I had to literally hold on so as to not to fall off.

On that note....always get down to ground level to change the straps on your stand. I was about three or four feet off when I realized I didn't have the base angled up enough. I redid the top and tried to pick the bottom up with my feet and disconnect the strap...which I did...and then the stand slipped off my feet....whoosh to the ground. It is also a good idea to have staps between the two sections..to prevent this.

Reid

I try to get at least 20, but it depends on cover. I've hunted in trees that are like telephone poles, although not my first choice sometimes the only choice and I've noticed if you get up 25 ft and move only at the right times the deer don't see you.