I'm kind of torn between 24 hours and 48 hours; 36 won't work because that implies a 'rotating' hour schedule on your part (or future organizers who have to move/update icons & actions, etc.).
I currently like the 24 hour plan as is; yes, there will be times when some of us won't be on for a day, but then again, WoE is supposed to be 'slow' to begin with. The amount of time it will take to move from one part of the map to the opposite side, doing certain quests, getting your army up to where you want it to be via leveling and taking skills, etc. Even with 24 hours, if you miss the literal day, it shouldn't impact your team (or your) progress in any way.
Of course, 48 hours also works, as it gives organizers MUCH more time to update things properly, cross-checking facts, preparing new things on the horizon, etc. It also gives more time to players to communicate and coordinate strategies, as well as try to get in touch with players who might not be as active/check to see if they're still playing.
...I chose 36 anyway. >_>