Thanks Goryn, for the answer.
Benroq, while i agree with you, that only the most dedicated players will be able to open level 6, I think 16h is a nice compromise from Inno. Yes, 12h or even 14h would be better, but if Inno's plan was to make money by making players spend diamonds to open level 6, giving us 16h is a good compromise. This way, the truly dedicated players will be able to open it without spending diamonds, and players with less time can spend the diamonds to open it, and Inno will still make money out of it.
In the meanwhile, for less ambitious players, doing the other 5 levels will be more manageable with a 16h timer, especially for players in different time zones.