Interesting question. I'm not sure of the answer.
Of course, you could always just shell out some money for a premium membership and not worry about it. You would automatically go on vacation at just the right time to avoid both timing out and over-using vacation.
I have an interesting situation shaping up as I try to ramp down my games prior to going on vacation at the end of this month:
I currently have one game on the go, and fully anticipate that it will be completed before I leave, and I am also between rounds in a tournament that I've advanced in.
Were it not for the tournament, I would simply complete the one game I have on the go and not worry about turning my vacation status on before I left, confident that I had no games in which I might time out. Because of the tournament, however, I will need to turn on my vacation status in case the second round starts while I am away.
My question is this: Does my vacation time continue to count down even when I have no games on the go? If it does, there may be a number of days, (or possibly even my entire vacation given the pace of the last couple of outstanding games in the first round of this tournament) when I am burning vacation time even though I do not have any active games.