Game-time. I like how FFXIV does it - roughly an hour for a game-day. It's not too long but not too short. I dislike the idea of having to be gaming at a certain time in order to be able to access something - it's not user-friendly at all. You could fix it so that the game didn't match the real-world clock, but just time progression pace... but I would not want to have to leave my game sitting for 12 hours so the sun can come up/down in order for me to progress with whatever I was doing.