It depends on the position, and the method you use. If you still have ideas, keep evaluating them. If you're just staring at the board, waiting for something to jump out at you, take a step back and look at it systematically. When you've exhausted everything that looks reasonable, start looking at things that look unreasonable, especially those that you were initially drawn to, but rejected for some reason.
Look at your worst piece, and how you may be able to improve it. Look at your opponent's best piece, and how you may be able to make it worse, or trade it. The key is not how long you look, but how many things you know to look at.
Remember that finding the answer is not important. Being able to find answers (good moves, good plans, identifying weaknesses) is the important part. It's less about the time you take than about the process you go through.
I've been working through CM for a number of weeks now and really enjoy it. One thing I'm trying to do is force myself to stare at the board for longer than I'm inclined to before clicking on a hint or other help. It got me wondering, how long do others look at the board before giving in to temptation and seeking help? (I think this has worked in at least a couple of puzzles. I've been stumped for several minutes before finally seeing the solution.) It seems that being willing to keep looking is key to improving at chess; I tend to be impatient and want to find the answer and keep going.