If you have 5-6 players and at least four of them know what they are doing, the '39 game is about a 14-15 hour game to Autumn 1945 But most games will end before Autumn 45. Less players or less experienced players will make the game go a little longer.
The 1941 game is more like 10-12 hours depending on experience and number of players. 1941 Pacific or Europe only is a little faster - about 8-10 hours. The Pacific theatre game is really only a 2-3 player game. The European theatre is ideal as a 3-4 person game. Worldwide is ideal for 5+ players, though we have played it many times with just 2-3 players - just takes a little longer - a lot to manage.