Etiquette Question
Different games go at different speeds, so there's no set time...if you were running at 4 posts a month, say, it might take 2 months to realise it died, whereas something where folk were making multiple daily posts and there's suddenly nothing for 10 days with no known reason? Probably dead.
If you're pretty certain the GM has plain vanished, just press that little minus symbol beside the game to drop it from your sticky list; should the game revive by a miracle, it will come back, and you can read through what you had to get back into the swing of things without keeping that mental 'bookmark' active and draining your creative energy.
If it's a slow game and you hit a holiday period and/or aren't sure if it has died, PM the GM. If you see the GM elsewhere on RPoL and they haven't logged in or posted to the game, try an RMail to ask if they are keeping it around for a reason, give notice of your quitting, or ask if they're going to delete it like a responsible game owner (beware of GMs who hoard games they abandoned without deleting, for the look of being highly competent with lots of active games on the Players Wanted board...they won't stick with whatever they're advertising, either).