l33t_haxx0r on 28 February 2015
Interesting one, with newer games and steam around its pretty easy to tell how long, but for older games not so much. Here are just a few of mine I am 90% sure about the times..
Demon souls - 500+ hours
Dark souls - 2500+ hours
Dark souls 2 - 1500+ hours
Ultima online - 7 years ^^