Pop Time measures the time from the moment the pitch hits the catcher’s mitt to the moment the ball reaches the fielder’s projected receiving point at the center of the base. Pop Time is a combination of exchange time (how quickly the catcher releases the ball, measured in seconds) and arm strength (velocity of throw, in MPH). Arm strength is measured on "max effort" throws, or the average above a player's 90th percentile performance.
The Major League average Pop Time on steal attempts of second base is 2.01 seconds. Average times are calculated with the following ranges.
Pop Time to 2B: 1.6 sec to 2.5 sec
Pop Time to 3B: 1.2 sec to 2.5 sec
Exchange: .4 sec to 1.3 sec
Read more about how Pop Time works here.