I've got a ~13 hr flight coming up in 2 weeks...not looking forward to that. I couldn't imagine 20 hours unless I had sleeping pills or a first class seat haha. Good that these options are available though. Longest flight I've done to date was 11hr. It does make a big difference being on a 787 though like the one used for that flight - they are so quiet, cabin altitude is way lower than in other planes, they get fresh air, and they aren't nearly as dry.
In business/first though I think I could easily handle any duration but I can never justify the cost on the really long hauls