Isn't the REAL question "How many days in a row are Tommy and Oliver willing to go without sleep?"
I would say that if you want to keep the numbers really manageable, make it 10 days.
Then again, the longer the DO lasts, the more expensive it will be, thereby limiting attendance (and making it more complex for the DO organizers). If you also factor in the flying time to get to and from the starting and ending points, you're really talking more like 12 or 14 days for most attendees.
I think that the first *A Mega DO was the perfect length of time, at the right price point (hint, hint re negotiating a good discount for business class on the longhaul segments
).