The Academy of Motion Picture Arts and Sciences, AFI, and the BFI all state that 40 minutes or over. The Centre National de la Cinematographie in France says 58 minutes or longer. SAG says 80 minute or longer.
I go to quite a few screenings, and one thing that I have noticed is that, for the most part, the shorts which get the best response are usually in the 5 - 8 minute range. Notice that I said they get the best response, not that they are necessarily the better product. Also, most low/no/micro-budget features that do well at festivals run 85 minutes or less. I think this has a lot to do with attention span more than anything.