The box plot shows all four quartiles of the data set. Each quartile represents 25 percent of the observed data.
Note that the rectangle between 0.5 and 1 second represents the 2nd and 3rd quartiles. That means, the 2nd 25 percent and the tird 25 percent of the mice population in this experiment.
Therefore, the rectangle represents 50 percent of 100 mice, that is 50 mice.