According to their own stats only about 11% of their eligible staff used it on any given month. They don't say how many days they give off but I'll just guess 2 as a middle ground. I'm also not working out differences between a 28 day cycle and calender month, or odds they fall on a weekend, so this is really approximate.

But (0.11*2)/28 means they're only missing about 1% of their working hours on average.

That 15-20% longer might be true, but it's not going to by the menstrual leave alone, it'll be by other woman bullshit.