What is the longest time you personally on your own free will spent playing a game that you HATE, and why.

No more than a few minutes here. I have felt bad not completing some games, but in all honesty, I see no reason to play them if I am not happy with how things are going. Some games you just have to write off as a loss and forget about them.

I felt obligated to play through Half-Life, so I did.

I kept hoping that maybe it would get better, but it only got worse.

I just picked it up, but TF2. I can not stand the game, but yet I play (not very well mind you.)

Dark Salvation. Though it's less, "I've gotten this far, why stop" and more "If I can get past this fucking god-damn jumping puzzle on the first fucking level maybe it will get better." I'm taking a break from it right now... but I still haven't gotten past the first level...

Dragon Age. I was praying it would be good after the first 4 hours, so I quit and got on Icewind Dale 2 instead,

Countless hours on BG2 and Oblivion. Eventually started modding (a lot) for both. Then realized that if I had to mod a game to such an extent just to enjoy it, then that game probably wasn't worth my time in the first place. I did enjoy the modding, tho. Not a total loss. ;-)

Spent about a week and a half playing DA2 before the awful gameplay drove me to shelf it.

Unless a game is truly godawful, I'll at the very least rush to the end to see how the story ends. Then I'll never ever play it again. An exception to this is Halo. I didn't hate it, but I got so tremendously frustrated with trying to learn how to dodge the Hunters that I just gave up. I'm a PC gamer at heart and I didn't have the patience for it.

Of course, give me something like Super Mario and I'll play it endlessly, frustrated or not, even if I'm not actually enjoying it.

As to the modding thing, some games require more modding than others. For me, some modding is fun, and enhances the enjoyment of the game. Simply being able to tweak things you don't like, or improve upon game mechanics, is huge. However, there's a point, as ddmuse stated, that modding becomes a chore just to make a game playable. For example, I made very minor modifications to Morrowind apart from the Code Patch, (which was just engine improvements, and stuff like turning off that annoying plastic-wrap look on enchanted items) and enjoyed it greatly. Fallout 1/2 I would have modded more extensively, but to this date nobody's figured out all the quirks of the system yet. So mostly I just tweaked the armor/ammo settings. I made more extensive modifications to Oblivion, BG2, and IWD2, but still enjoyed those tons, and played them endlessly. Fallout 3 reached the breaking point for me and my modding. I was spending half of my time digging through game files, fixing bugs, and tweaking settings instead of actually playing the game.

I'm playing Terminator Salvation right now, which is terrible. I mean really... terrible. Not sure if I will finish it but I have got to level 4. Might play it again, might not... not sure why I would, just to finish it and get my $5 out of it I guess.

Grin's game before that Bionic Commando was just as bad, but I quit playing it after an hour or so.

Completed the whole game of Alundra 2. This game has so many problems. I played through the whole thing just to be able to say I finished it.
For the record, the first Alundra game is pretty good.

If I don't like it, i don't keep going back.

Reviewers on the other hand.......

....poor bastards.

I once played this MMO that I loathe for few months because of this girl.

When I found out that I have to have the solo this boss, that was end of it. :( Never see that girl again.

If I were to discover that I HATE a game, I'd stop playing it right there. That's a lot of passion to feel over an amusement though. I don't think it's ever happened but with games like Super Columbine Massacre RPG existing, I wouldn't rule it out.

I'll play an uninteresting or meh game if it's well regarded or does something innovative just to have an opinion on it. I don't have an upper limit but 5 - 10 hours sounds typical. Uninteresting games without any apparent redeeming qualities go to the bottom of my backlog abyss.

I've tried to play first Halo through twice, so I've spent ar least 10 hours with that. Then I pushed through Diablo 2 during 4 year span all because of the cinematics.

It would be disingenuous to claim that I outright hated it at the time, but I kept on with World of Warcraft for several months after I was burned out on it for the sake of my raid guild, since I was main tank and didn't want to ditch. Happily, the first expansion was about to arrive, which meant a hold on raid activities and an excuse to terminate my account.