I don't really know, I guess I don't tend to spend more than 70-80 hours on a good game (assuming it has that much play time, or else whatever its playtime is), some games i'll keep coming back to like L4D, baldurs gate, kotor, counter strike: those are going to have a few hundred hours each, I guess the most possibly is on one of the last three, i'm pretty sure I spent at least 300 hours playing kotor, at least, and as for baldurs gate..
well lets just say I beat it with every character type at least twice (varying sex or alignment, or both) plus many more times besides with different parties or even solo on three or four occasions. I beat it solo as a thief, a warrior, and a mage that I can remember... of course when I say i beat it I mean i got to the last fight, you can't win that fight solo and certainly not as a mage as the main baddie is completely immune to magic!
Anyway so i'm guessing 500+ on baldurs gate.
its very difficult to even give ballpark estimates for some of these games though because, to be frank I played baldurs gate over 10 years ago and my memory simply isn't that good
as for counterstike, well i've been playing it on and off since version 1.4, I remember when version 1.6 came out (along with steam) I took a few weeks off work and played solidly the whole time (much to the annoyance of my then girlfriend).