I think if Stewart makes smart career moves and plays to her strengths, I think she can shed that Twilight image within a couple of years. Ditto for Robert Pattinson -- and his next post-Twilight movie is a David Cronenberg movie for God's sake. You don't get more serious than that... and the franchise's success gets doors open for the actors to do passion projects.
Leonardo DiCaprio was able to shed that boyish Titanic stereotype in less than a decade. People see him as a serious actor and capable leading man, rather than a teen idol. If he can do it, the Twilight actors can.