I'm left wondering if you've never been stuck in a game with a griefer who is hoping to wait you out.
I have, but I didn't get the impression that this was what the OP was asking about - it seemed to me they were thinking of a time limit in terms of general tournament logistics, not "how long before we should time-out douche-bags". My opinion on that is inline with polokus: you should have human intervention. Those kind of things are judgment calls, like unsporting behaviour in a football match. Martial arts have rules in place that mean you get docked points for lack of attacking play, but it still takes a referee to make that call.
Playing computer games has made people get used everything being black and white and the idea if a player can physically (digitally) do that then it's all kosher and fine, and if it's not fine then we need to make sure the player physically cannot do that in the game. Well, often we have to play by that standard because there's no other way to enforce it, but for me that's not ideal at all, it's an unfortunate practical limitation that should be avoided if possible.
If people going to be douche-bags then they'll be douche-bags. If it's not by slow play then it'll be something else. Don't change the rules just because some of you haven't got the patience
in a tournament to out-wait a slow player you've clearly beaten. It might be sporting to concede, but you don't see tennis players doing that do you? You need to find some way to notice it warn/disqualify them (eg take the time-hit on a long game, but get game logs with time-stamps, or look at the chat logs to see if they're chatting shit at you the whole game or whatever). In a face-to-face tournament this just would not be a problem at all, it would be so easy to spot.