There have been rumours that Charles has been the regent for last 5/6 years of the queen's rule. I think that the reason she didn't step aside was because of the jubilees. Face it - big business for the Royals and the UK - touisim etc.
However even with the best health care, I don't see Charles making his silver jubilee.
I think, if his health lasts, he might chuck it after 10yrs. By then the Boy George will be at University, Big Willy and Slim Kate will be early 50s and should have a good run of 25yrs as monarchs.
Plus they'll need to have some big royal event to keep people interested. The past 20yrs have seen loads of events - jubilees, weddings, Olympics, and now a coronation. They'll need something to stop them fading into the back ground.