Unless you are literally 100% sure you've lost, don't resign.
99% is not 100. Remember that.
Percentages are a poor way to represent one's assessment of a chess position, especially for people rated <2000. Exactly what would constitute the difference between a 75% chance to win and an 85% chance to win?
The question is qualitative. Do you have legitimate counterplay? Fine, play on. Is your position completely out of gas? Are you going to have to rely on a serious blunder from an opponent that is unlikely to make one at their demonstrated skill level? Then resign, and play another game. If you have a possible swindle in the form of sacrifice for a perpetual check or the like, by all means go for it. If your idea of a "swindle" is to keep moving your lone king around and hope your opponents stalemate you with their Q+R, then you're just being an annoying dweeb...at *least* as annoying as the players that will promote 3-4 queens and try to embarrass you for not conceding a lost game.
All i hear is someone complaining about people not resigning because he has stalemated in a completely won position before. Never give up.