If you start noticing how long it is, then it's too long. If it is good and well written, you won't notice the length either way. I bank on spending a 2 minutes reading a hub. I would say that's 600-700 words for me. So if you want an actual word count, there ya go.
My longest hub is 1360 words, but it's a short story, with lots of dialogue. Perhaps it reads as fast as a 500 word, technical hub.
Here's the link... http://hubpages.com/t/2bccfc check it out if you'd like.
Happy 2012! Have a great year & I wish you success!