There doesn't seem to be a consensus on this point ...
Blocked from Posting on Resigning
Customary resignation response
Is it rude to keep posting about being rude for not resigning?
Kobayashi Maru (Hopeless Situations)
Letting time run out instead of resigning in live chess
LISTEN TO ME NOW!!!! and understand me later.
Not resigning “when you should”
Problem when resigning telling you that you've resigned
Resigning Attitudes - Let's Vote
The amateur mind in the endgame
Was GM Timman correct in resigning?
What's the average rating for resigning a game?
what is everybody's opinions on when to resign? if you see mate coming your way should you resign or play till mate? if you drop major material should you play on or resign? what is proper etiquette?