Yeah far too many times, ignoring all the Amiga, C64 and Spectrum when I was a kid - strictly talking "modern" pc times.
I did a lot on Half Life 2 when that came out, deathmatch was on constantly - never monitored hours though but it was a LOT.
Battlefield 2 (who didn't) couldn't put it down - at a guess probablly 12 hour stints.
Doom 3 - I got it when it came out, my partner went to bed at 10pm and I literally lost track of time, I went to bed at around 7 a.m just as she was getting up.
WoW - raiding and pvp, so many stupid hours - I think the most was 26 hours straight.
I won't specifically mention console games as it's in the pc section but there were a **** load of stupid hours put into games, sometimes alone, sometimes with mates - where we would complete games on every difficulty setting one after the other.
Now, I only play an hour or two a night at the most - although I would imagine when BF3 and Swtor is released this will change drastically.