The play clock is 40 seconds and starts as soon as the previous play is over. The game clock stops for the first down and starts again when the ball is ready for play. That is usually going to be a 6-12 second gap. HS uses a 25-second clock and it starts the same time the gabe clock is started after a first down usually 12-15 seconds. The end result is about the same. College can get the ball set a little faster due to 7-man mechanics. The benefit in NCAA is the timing of the play clock routing is much more consistent.