"Time" magazine created an online calculator to do it. First you estimate the amount of time you spend on Facebook each day, which is 17 minutes for the average person. Then it scans your entire Facebook history, and spits out a number.
Chances are you'll be shocked at how high it is. Because even if you're not a huge Facebook user, you've probably wasted DAYS or WEEKS of your life on there. You can check it out on the "Time" magazine website.