I'm learning the hard way that writing long hubs actually deters many readers somehow, and since many of my longest hubs get the lowest responses, and numbers, my recommendation is to be wise about how lengthy you wanna make a single hub.
It's a matter of whether your purpose for such a hub is to get read by more readers, or to merely exist amongst the many in the hubpages archives, and I look at it that way, because many peoples hubs simply appear to go unread possibly due to, over doing it I guess with lengthy hard to read hubs.
Now if you hub topic can draw the crowd, and have powerful content to impact them with & fully illustrated, and can keep them viewing as well as returning, then you might just get away with having so many words, and so much to offer. Its to each ones own I guess, and user discretion is advised.
Awesome Question here, thanks for asking it.