The Monk Temple

[Viridiana]

Any time Demo is not already on the target, and the target is expected to live for longer than 15 seconds or so, Demo is the better choice. More power for your GCD. Plain and simple.

Twelve seconds, absolute maximum, and that's assuming you don't get the attack from behind. Nine seconds is the maximum if you're properly executing the attack from behind, and that's also assuming that you apply right after the global tick so that it takes a full 3 seconds for the first tick. More realistically, somewhere between 6 and 9 seconds is the time it takes for Demo to beat Snap.

Oddly, some people seem to think this means you should replace all your Snap punches with Demo, though. That's a horrible idea, and they should be ashamed. >_>