How Many Seconds of Video Equals Fair Use?

(Warning: lots of scatological language) 

Dear Rich: I've created a 12-minute video that consists of short clips from 45 different movies. It's something like a cross between a supercut video and Christian Marclay's "The Clock" or Vicki Bennett's "Gesture Piece". From my long-time reading of Dear Rich, I assume that the video will probably (1) fly under the legal radar or (2) be flattering and harmless enough to not offend the copyright holders. But on the other hand I assume I'd have to rely on (3) a fair use defense if any copyright holder objected to my use of a clip. Is there any better news than that? Most clips in my video are up to 5 or 10 seconds, a couple are 30 seconds long.  Is there any kind sense of how long a clip might be before it triggers a legal issue? We're often asked that question, or its equivalent -- how many seconds of video qualify as a fair use? (And we're asked similar questions about music and books.) The answer is the same for all media; there is no fixed limit that is guaranteed to be safe. The analysis is always case-by-case and usually focuses on three factors:

• how long? (shorter is always more likely to be permitted), 
• how important? (the less important the clip is -- that is, the less it qualifies as the "heart" of the work -- the more likely it is to be permitted) and 
• is it a transformative use? (the more transformative your use, the more likely it is to be permitted).

One way to assess this for yourself is to review some audiovisual cases in which portions of films were used without authorization. We've collected a few at this site. In general, you're correct that you're likely to get away with such uses as the movie studios apparently don't find these snippet collections as threatening to their bottom line. That is, they are more inclined to go after BitTorrent sites and those pirating complete movies.
BTW Dept.  Thanks for getting us to explore the endlessly entertaining world of supercuts. We wanted to include this DeNiro supercut, but it seemed too violent and language offensive for our audience so we opted for the above supercuts classic (which is just language offensive).