On Oprah David Blaine did  17 minutes 4 seconds. 
http://www.huffingtonpost.com/2008/04/30/david-blaine-breaks-breat_n_99407.html
I'm curious how his Zen method works thrashing against rocks and currents upside down in a kayak.

I wonder how he would have done if he wasn't allowed to breath pure oxygen for 30 minutes beforehand.  It brings to mind a method I read about pearl divers and something that's helped me.  They take 3 deep breaths before they go down to fill their lungs with good oxygen.  When I remember that before that possible roll it helps me.  It increases my time at least 50% if i try w/in 5 minutes or so.  Problem is that rolls often come unexpectedly.

I start deep breathing whenever I am freestyling or running hard rapids and it does seem to help. It is pretty much automatic now, if I roll I always seem to have a comfortable 30 seconds or so stored up. 30 seconds is a long time under a river, try it sometime in an eddy or slow moving section.

I breathe heavy when I see women in kayaks, neoprene, polypro, or any combination of the above!

But seriously, I agree with D4 that I usually seem to end up with an adequate supply of air to work with while capsized, and don't really even think about it. But 40 seconds is fairly easy to hold it, and often over 60 or 90 seconds.

Once at Bozeman Hotsprings I held my breath  (after alternating between the hottest and coldest pools, then submerging my head in the 50 degree pool ) for 2.5 minutes