It depends on the watch. The GPS tracking function alone is a serious drain on a rechargeable battery but add in things like high-resolution color touchscreens, music streaming, and other battery-sucking features, and it can't be overstated that you'll want a watch with enough juice for the long haul. Good battery life is important for all your gadgets but it's crucial for running watches. If you're running a marathon or in the midst of a long training session, the last thing you want to worry about is your watch dying on you.