If you want to make it through the QAP, I would suggest at least 500 words. Check your competition, because you need to beat them to rank well at Google.
I do have an old short hub that gets a lot of traffic, but Hubpages is now recommending long hubs. They are saying 2000 words to rank well. For some subjects this would be just too much fluff. For others it wouldn't even be adequate. You need to use your own common sense.