To try and decrypt a couple of the posts here, the length of your hose will depend on how you intend to deploy it.
A lot of recreational divers will keep their octopus secured with some sort of device, ready to be deployed to an out-of-air diver.
On the other hand, most technical divers, (and also more and more recreational divers) will keep their octopus on a piece of bungee cord around their neck. If their buddy needs air, they will donate the regulator from their own mouth, and switch to the octopus around their neck.
In the first situation, you'll want a longer octopus hose, as you suggested 30"-36", which ever you feel more comfortable with. And probably a shorter primary hose.
For the second situation, you'll want a longer primary hose (often 5' or 7'), and a short octopus hose (around 24")