IMDb already has the movie and episode duration data. It would game-ify the service a little and provide an extra incentive for people to rate shows, so to keep track of progress, which might increase user participation.
It already exists. Here is mine. You have one to if you are a registered user. If you go to my page below, it will compare titles we have both rated. It will show the like titles and whether or not you rated them higher or lower. But it will not show the hours however. Impossible to calculate. With commercials. Without Commercials. With Commercials with increased speed presentation. With Commercials edited (trimmed) view time. Formatted for television to fit within the space and time allotted with commercials. Too many variables, and don't forget about DVD Times vs. original and Director cut times too.