There is no rule saying that a player has to resign for any reason. As for ediquette, I'm not so sure. Personally I don't mind whether someone resigns or not. I am not above losing won games, so I can use the practice of finishing the game. If I do lose, well I certainly have something to analyse. And if I win, well this is just exhilerating. I usually resign games once I see no hope, but even this may mean different things to different people. Playing out a game which is"lost" from my position on the board is certainly not very much fun, even if your opponent blows the position it is still somewhat of a hollow victory for you because you know that you should have lost (if you opponent knew what they were doing).
Time to resign
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I've recently had a talk with one of my opponents: I had over a queen of advantage and after a couple of moves I proposed him to end the game and begin a new one. His point is he's always giving a chance to the other side to screw it up, even if he is very behind. I'm not sure about official chess etiquette, but I don't think this is very polite.
What do YOU think is the right time to resign?
(This is definitely NOT a complaint -- I just want to learn the habits.)