Answer: The maximal time is 100s.
All ants are identical. Consequently, the "collision process" between two ants can be greatly simplified if we think of them as simply passing through each other.
Now it is much easier to reason about the dynamics, because the ants do not influence each other at all. The maximal time that a single ant can be on the meter stick is 100 seconds (length divided by velocity).
Consequently, this is also the maximal time that any number of ants can spend on the meter stick. There are several arrangements that lead to this solution. For example, all of them starting on one edge, heading towards the other.