Puzzle 30 - Ants on a twig

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After an unsuccessful attempt in trying to translate Upanishad (aim high, right?) and not very successful attempt in writing Sanskrit tutorial posts, I am back to riddles and puzzles.

But this time, I really do not have a solution. Like many, many puzzles in life. So if you, dear reader, find a solution, please send it in the comments. 

I have "borrowed" this puzzle from Quanta magazine. And there was no solution provided.

 Imagine a narrow twig spanning two mounds on an anthill to form a bridge 100 inches long. There are 10 ants on the twig, evenly spaced from the left end (let’s call this coordinate 0) to close to the right end (coordinate 90). You can ignore the lengths of the ants themselves. The five ants on the left are facing right, and the five on the right are facing left.

 All of the ants start walking in the direction they are facing at a constant speed of 100 inches/minute. The twig is so narrow that when two ants meet, they turn around instantaneously and walk away from each other at the same speed, exhibiting the kind of frenzied behavior typical of ants. When an ant reaches the end of the twig, it descends to the mound it has reached. 

How long does it take for the twig to be free of ants?

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