Answer:
Due to the popularity of the movie "Mr. Smith goes to Washington", many believe that as long as the person who has the floor does not sit down, keeps talking and stays awake, that it can go on forever.
In truth, a variety of laws passed in the 20th century limit it by means of something called "cloture". This allows a given percent of the House or Senate to vote an end to it. 3/5ths in the case of the Senate.