I get where you're coming from. The DF can be unpredictable, and certainly eats up a lot of time just setting the damn thing up. In an ideal world, you'd just resign yourself to other endavors and withdraw from the DF once it became apparent you couldn't have completed the instance it was setting up. Now, I'm not criticizing you here - Its hard to gauge how long a dungeon can take (moreso when it's your very first time in it :P)

The DF gets its share of ragequitters these days (people who bail if they die or aren't happy with the grouping) and just to not leave that sour aftertaste with the remaining members, I'd probably explain quickly that RL (Real Life) issues came up, and I have to leave. Most sane people will understand the concept that RL is more important than FFXIV, and though you may piss some people off - thats just an attitude-problem on their part.

In short; Just calmly explain that you are sorry, but you have obligations outside FFXIV and you have to leave.