The Chicago Tribune has put together a great, if depressing, graphic showing all the steps required to fire a tenured but ineffective teacher in the Windy City. The short version? It takes 2-5 years, and as many as 27 steps—which, according to the Tribune, is why many school principals don't even try. See the whole thing here.
In 2006, Reason published an illustrated guide to firing a teacher in New York City's public school system.