RB_CH01_Q020. 10000 ants on ruler

I have dropped 10,000 ants randomly onto a ruler that is one meter (i.e., 100 centimeters) long and oriented to point north-south. The ants are of very small size and mass. Each ant walks at a steady pace of one centimeter per second in a straight line parallel to the long edge of the ruler. Their initial direction is randomly either north or south. The ants are all from the same colony and possess an inherited vision problem: they have peripheral vision only. This means that they can collide with each other if they meet head on (although very small, they are large enough to collide). If two ants do collide head on, however, then they each turn around instantly and head back the way they came in their steady pace. With so many ants in one small space, a single ant may experience multiple collisions before it walks off of the ruler. So, how long must you wait to be sure that all the ants have walked off of the ruler?