If you are playing someone <1800, you'd be surprised how many chances they'll hand you to come right back in the game so, as a rule, you should NOT resign at that level.
For higher ratings, it's more of a personal preference - some people resign as soon as they are worse, others will play out games till checkmate no matter what, most will resign when they think their position is "hopeless" (as judged by them) and there are some unwritten etiquette rules (e.g. allowing a particularly beautiful checkmate, etc.)
my thought is to continue playing--there's always a chance my opponent may make a mistake, or maybe i'll get a draw. some have told me it's polite to resign when it's obvious i can't win...opinions anyone...?