I feel a successful animatic should have fluid movement between scenes, as in one scene leads nicely to the next; whilst making sense. The viewer needs to understand what is going on or else there is no point. I think 30 seconds or even less is enough time to provoke a reaction/emotion from the viewer, this is something I would be aiming for in an animatic.
A tutorial video I have found below, was helpful.
The band Gorillaz have some good animatics that have been produced for their music videos. These animatics are perfectly timed and present clear ideas/storylines. This is something I feel can be achieved even within the space of 30 secs. The video won’t allow for embedding so here is a link. 🙂
Hellman’s have an advert out which kind of combines 2D and 3D animatic approaches. This is the best video I could find online .