Calculate far enough to reach a conclusion (i.e. a "quiet" position) and then look one move farther, OR, as far as you are able to calculate, whichever is shorter!
Calculation

well, normally, you should choosae candidate moves, then you should calculate 3-4 moves ahead PER candidate move..
I usually take 2-3 candidate moves and 6-10 moves ahead in each of those.
We should distinguish between online chess and over the board. Obviously in online chess you can go much deeper -- I've gone as far as 20 moves out. But I can't do that in my head. I have to write out the variations in Excel.
In Chess it is Important to Calculate and to be a step ahead of your opponent, but how far is far enough, and how far is to far? Also how many variations should you calculate, and to what depth? I'd love to hear all of your opinions!