Guess it all really depends on who's writing them, I guess.
Come to think of it, when it comes to writing mail like that, I RARELY get a response from whoever it is I'm writing to. In 2004, I wrote to Kevin Clash, and in a matter of weeks or so, I received a wonderful, and rather colorful autograph of him and Elmo; meanwhile, in 2005 I wrote to Kathy Greenwood, and a few months later she not only sent me an autograph, but she also wrote back. Other than that... no such luck.