Valve has just today added a new feature to the Steam community profile page, namely total playtime. This addition has been awaited, since a Valve employee mentioned it long ago (HERE), and, finally, it's here.
Normally, you only have the option to view playtime for the past two weeks, but since March this year Valve has been recording gameplay hours of your games through Steam. Now, when you look at someone's profile under gameplay stats, you'll see e.g. "9.7 hrs / 157.7 hrs" where the first number is from the past two weeks, and the latter is total playtime since March. You can also look at people's gamelists and see how much they've played their games.
Here I should note that total playtime also includes idle time.
Here I should note that total playtime also includes idle time.