How did Gerry know the zombie transformation time was 10 seconds?

Question

In World War Z, we come across a zombie apocalyptic plot featuring Brad Pitt as Gerry being the main character.

The movie starts off with sudden panic arising in the streets, and then an explosion, followed by terror and screaming people being followed by mysteriously behaving people quickly discovered to be zombies. It is apparent that no one would have stayed behind to time how long it took for someone to transform into a zombie, which would lead to an easy death of the observer.

So after Gerry fights off the zombies on the stairs, he's on top of the building leaning over the edge of the roof appearing to be counting, with his family behind him watching in fright but unaware what he was actually doing. He soon excused himself to have gotten it in his mouth. As seen:

How did he know about the 10 seconds and when did he find out

Answer 1

How did he know about the 10 seconds and when did he find out

Because he watched it happen earlier and counted.

There is a scene early in the movie when Gerry first encounters the spread of the zombie disease. He witnesses someone being infected and counts how long it takes for the infected person to be transformed into a zombie.