The Last Ant Off

A straight pole is exactly one metre long. Several ants stand at arbitrary points along it. Each ant chooses one of the two directions and walks steadily at one metre per minute. Whenever two ants meet, both instantly turn around and continue at the same speed. An ant that reaches either end falls off and takes no further part. You do not know how many ants there are, where they begin, or which way each one initially faces. Assume their turns take no time and that they never stop. What is the longest possible time you must wait before you can be certain that every ant has fallen from the pole?