How do you think it really works?
Since competition shows are usually up to the producers' discretion as to who wins, do you think that this is more up to the producers or to the "stars"?
I'm thinking, and of course could be so very wrong, that the producers contact some celebs, and see if they want to be on the show. How much they want to appear could have something to do with how long they are on the show. Or it could just be how long they want. I just don't really think that the audience has any say.