There’s a rather slick website called “tiii.me” which uses an easy interface to calculate the amount of time you’ve spent watching a TV show, or multiple TV shows added together. You simply type in the name of the show, and the number of seasons you watched, and through some sort of database (IMDB?) it goes through taking into account the episode # and length to come up with a figure.
It aggregates too, so once you start, you can keep adding shows until you hit a big total. I kept adding shows until I reached about 100 days, then I just started getting depressed so I stopped. Turns out I spent over a solid week watching ten seasons of Smallville alone. Yeeesh.