Depends on how you want to monetize your plays. If you're running 20 mins, you only get 1x pre-roll in. If you run that as two 10 minute spots, you get two pre-rolls. I've been producing for YouTube for a while and only run something longer than 6 minutes if I already made money on it before it's posted online (ie: product placement or burning in my own mid-roll ads.)

If you're doing it for the love, use Vimeo. Otherwise, don't really understand why anyone would use that platform since the T&C don't allow you to monetize the plays (running ads disguised as entertainment aside -- such as, the awesome Iceland promo.)

Think of the network television model... a typical "30 minute" program is 22 program minutes broken into 3 clearly defined acts of about 7 minutes each. People these days have very short attention spans. Music is even worse ... it's becoming increasingly rare for someone to actually listen even to a 3 minute song in its entirety anymore. After about 30 seconds for the typical listener the finger goes to the next track button on the iPod.