Anything really. Perhaps its a personal emergency crisis (like a family death, health scare,etc.) Maybe it's because you don't have time, are burnt out, or are suffering from writer's block. If I were to take a break it be for a combination of reasons, mostly being burnt out, no time, writer's block, and no desire/motivation to write. I don't know how long is appropriate, it is up to you. Its mostly how quick you get that desire and urge back.
A way to keep a level of contact on your sub-domain fresh and active is to log on at least once a week and perhaps answering questions and going on forums. There isn't much of a commitment as oppose to writing hubs and it can make you feel inspired to get back into creating hubs from visiting an commenting on forums and questions. :)