Q u o t e: Once players get to the point where they can farm a boss, it will take maybe 5 minutes to kill him. Nobody will make me believe any different(excepting Bashiok).
I think it would be hard to kill Baal or Diablo by yourself in under 5 minutes without a hammerdin. Baal would take even longer if you were incapable of teleporting/running immediately to him due to maphack. Diablo 3 bosses could easily take half an hour to fight to and kill especially if they place waypoints well, have events leading to a boss, and create interesting boss fights possibly with multiple phases where they use different abilities (think of WoW raid bosses or even certain Final Fantasy bosses).