We have talked about this in the past, basically we feel that most people will not bother to write in the duration times. So it would work but only for x number of videos. However, if the various video services start offering an api to retrieve total running time we would happily implement such a feature.

I would certainly use the feature for all submissions (in my usual long ones of course: but short too). How about an optional field when submitting which adds an icon/time in full view (appearing like the NSFW, but below) with the time? It's a shame the usual suspects (YouTube/Google Video) don't add the full running time to the preview interface.

again, we think lots of people would do it, but I strongly doubt everybody would. As a general rule of web site maintenance it is best to design for the lowest common denominator.