I would assume they would pro-rate whatever time is left on a subscription and only charge that amount until the point of no return (server-down). It would be a simple calculation. The "monthly" plans for FFXIV are in fact based on 30 days I believe, rather than calendar months, so it wouldn't be difficult for a simple algorithm to do the math on that.

Just a theory of course. Considering all that has gone on with this game, some time free would be nice too, but I'm a realist at the end of the day.