As many different answers as there are hubbers.
Somewhere around 300 words seems to be the minimum for HP, and such hubs may be classified as substandard.
A better range might be from about 500 words to around 5000. I prefer to write between 1500 to 3000 word hubs myself, but these are on the long side from what I've seen on HP. Not particularly unusual, but longer than average.