I feel that a short needs to be less than five minutes to keep someone's attention. There are exceptions for going longer, but generally no. The beauty about a one minute short is that you're not asking someone to commit a large chunk of time to watch your film. Even if they suspect it might not be a great movie, you can generally get someone to give up 60 seconds.
technically in some places 45 minutes but what would you actually do with a 40 minute short?
its unlikely to get accepted anywhere since its too long to fit festival scheduling (why accept a 40 minute movie when they can have 4 x 10 minute movies or 8 x 5 minute movies or 40 x 1 minute movies) so unless its mind blowingly fantastic then its kind of the equivalent of a 3+ hour feature, yes it exists but its stupid to make your film that long unless you have serious bank-ability like Coppala, Jackson or Spielberg and they all started much smaller