I personally believe that the day/night cycle should be long enough to feel like a significant amount of time has passed in-game. I don't want the time very short (10-30 minutes for a full day/night) nor do I want it to be excessively long (2-3 hours for a full day/night). As long as the game feels reasonable enough to feel that time has indeed passed. then I'll think it the day/night cycle is fine. That said, this might be a stretch but it would be incredible for the game to use the system's internal clock as the way to tell time. I can understand people not liking the idea because that would mean in order to view the game at a specific point in time they will have to be available in real-time, however I personally enjoy cycles like that as it made you feel like you are in the game in real-time and that in a way allows the player to immerse into the virtual world even more.
I don't know, i know the game will look beautiful in both night and day and if there are night quests like Poe hunting then it'd be nice for it to last a little longer than the day like it did in TP.
I think the cycles should be rather long though. It didn't bug me so much back in the day but whenever i play OoT now the day seems to turn to night and vice-versa so quickly, but i know its a scale thing, a faster passage of time for a smaller world to make it feel like its bigger than it is. Thus by that logic Zelda U's night/day cycles should be pretty darn long, weren't Skyrim's an hour?
Either way though we definitely need a way to change night to day when we want. It was a bit of a bugger in TP to wait until nightfall sometimes and if Zelda U's cycle is gonna be longer then its going to be even more of a chore so we need an item or something to do that.