
Well the title says it all, right now you can see how much did you play your games in total. Unfortunately you can see the total gaming time only since March 2009.
Here’s a quote of what Mike Blaszczak (Valve) said on Steam Forums:

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Rating: 4.2/5 (6 votes cast)
Steam Community: Total playtime introduced!, 4.2 out of 5 based on 6 ratings
cool feature.. tells to the world how much of a looser you are xD
Not one of the most important things I would like from valve but still very cool.
March 2009 isn’t bad, i didn’t even expect to see statistics before the implementation of the feature.
I sure hope the “new” Steam software will be what we’re all waiting for.
Thanks Valve for this little piece of candy!
I actually have x-fire for that sort of thing…