The Leaderboard could update at any interval we require however we find that having it update at decently timed intervals such as every hour, day or week creates a micro-news event that can be used to notify and then motivate each player ('you're now #3 on the leaderboard, up 2 places in the last hour, keep going!') for example.
The technology can work with any interval, but having the leaderboard as "always up to date" loses this sense of a news event each time the leaderboard is released. It is the associated player movements that people like and engage with. Having an interval has an added benefit of leaving time for the leaderboard manager to moderate unwanted tweets or players too.
If you'd like to see more flexibility leaderboard release intervals then please make your suggestions and case for it in the feedback section.