However, we must all remember a few basic premises which I think Adrian is highlighting:
1. Humans react to changes – this is why we don’t notice a tree growing, but a turn indicator on a car flashes to make us notice.
2. Timing is important – Another human trait is boredom. Whilst standing in a queue (line!), people are looking for something that it of interest to them, so although the ‘dwell time’ is longer, the attention span is much shorter. Again, during television advert breaks, when the person is relaxing they are much more appreciative of the 30 second long adverts – but of course these dont work on OOH.
3. It isn’t televsion – although I said a comment above comparing it to televsion, remember the OOH is just that – out of home. People have less time and less patience when not in the comfort of their lounge – this is why interactive kiosks shouldn’t just show websites on them for the same reason.
3. Humans react to changes – yes I know I said that, but (when legally possible) rather than just changing the content, make the content change – use Flash animations, interesting videos, or just transitions between the content images to re-attract flagging viewers.

Basically, in our experience 30 second slots are too long (especially if matching a TV campaign and reusing TV commericals), and if the overall playlist is long enough to match dwell times, and the content is interesting OR animated, any of the 7, 10, 12 15 or any size slot should be appropriate. The industry just needs to standardise so that (a) advertisers can be sure what they are getting and (b) so that animated/video content doesnt’ need to be repurposed/edited.

Also… is it me, or does a 7 second slot = more advertisers = more income in the same amount of time?