Should you meditate with a timer?

I currently do 15 minutes of meditation every morning. I use a stop watch. I see some people do more like 30 minutes. I was wondering how they kept track of time. Or is it roughly 30 minutes? I find that if I don't use a timer, I find myself thinking to myself during meditation, "I wonder if it has been about 15 minutes" and it seems distracting. I've tried meditation without a timer, but I guess the question is when do you feel you should cut it off?