The horizontal axis represents the exercise time in s; The vertical axis represents the distance in m; The trajectories of two people are two straight lines, so they all move in a straight line at a constant speed.

Among them, the movement time of A is 3s and the passing distance is 30m；; The moving distance of B is 30m, and the time taken is 3s- 1s = 2s. So B just caught up with A while moving for 2s.

So the answer is: 2.