I honestly think it depends on what game you are playing, there are no hard nad fast rules, but its not based on time. For me, I give up when the story sucks, (Infinite Undiscovery), its too hard (Mirror's Edge), badly designed puzzles (Myst 4, couldn't even play it with a walkthrough), unlikable characters (Infinite Undiscovery), or when I am enjoying it too much to want it to end (Albion, FFVII) but in the case of the latter, its because I want to play it again, not because I plan on never playing it again. I quite often don't finish games, but I am young, so I think I probably will get round to it some day. The only game I have ever abandoned and sold without finishing is Bully. That game just made me feel dirty.

I normally do some considerable homework before playing any games. That said once I decide to install the game then it's all the way till the end...no matter how bad it is. Why? Because I made the commitment during my 'investigation' phase.

However there are some games that just last forever (e.g. UT2004). When to quit? I don't know however rest assure that when I post up a review on a game you know it's one of two reasons:
1) I have finished it from top to bottom, left to right and up to down.
2) I have invested around 40-100hrs therefore I got a real good feel for the game.

Yet there where some games I thought 'That's it! It's too damn impossible to finish!' (e.g. Redneck Offroad Racing) yet being stubborn person at times, I gave those games a good rest and whatta ya know the difficult parts seems to be easier when coming back to them.

I'm always happy to talk to others who enjoy good old games, thanks for the add! I haven't been around much here lately though myself.

Old games shouldn't have to be all about nostalgia; some of them remain unsurpassed in greatness til this day, especially the computer games!

To answer your question: I usually quit if the game makes me feel sad somehow, if it makes me ask myself what i'm wasting my time with, usually because of flaws in its gameplay, or if it has a silly story, unconvincing/bad atmosphere, and not because of how much of my time it occupies - I can always turn a game off and go back and play it at a later time. But I usually try to stick with a game long enough to make sure that I'm not missing anything, a game could start out not being very impressive at all, but grow on you as you keep on playing it.

Some games I will probably never abandon, like Master of Orion, or the terrific board game Blokus (available online at http://www.blokus.com) as they have close to infinite replay value, and are a blast to play. Other games I keep coming back to now and then, the great adventure games of the past, great shooters, sports games, etc. I could list more examples but that's what my ratings are for: to keep track of the games I've played - almost the only thing I use this site for these days. It's too bad I can't remember all the games I've played throughout the years well enough to be able to sum them up in numerals - and I still always have a hard time doing it for the games I'm very familiar with, but... it's for my own sake.

Oh yeah, and some games I guess I'd prefer to remember rather than replay. I have replayed many games that I've been very unimpressed with the second (or third, fourth, etc.) time around, 2, 5, 10 or 15 years later. Well, that's nostalgia! But if one remember something as being good that's probably also the reason to go back... I try not to be blinded by nostalgia. And I believe there's less of that involved when I discover games that I have not played at all before. Or it may be because I know and prefer a certain kind of gameplay.

Have fun with the miggy games! I sold my Amiga computers many years ago (I had an A500, an A2000, an A600 and an A1200T), but I've still got to say that it's one of my favourite platforms ever, and certainly not only because of games.

oops, long post...

When? Well, it's when I realize that tonight is th last night before exam.
Sometimes, whenever my eyes get worse or they hurt or i have a headache or I feel like vomit or I want to sleep badly or the game get boring or after i throw my controller out of frustration and it breaks and i have to turn it to GAMESTOP telling them that it's not working anymore.

I stop playing a game if It's boring, and when I don't like the characters in it. I don't really care how much time I spend playing a game.

I like to give a game at least a hour to see if it improves, but there's no exactly science to it. If I don't feel like playing it anymore, I just don't.