Because the four turtles constantly change direction with the target they follow, the final crawling trajectory should be a circle with the center of one side (such as AB) and the radius of half AB, and take a quarter of this circle, that is, the arc OB(O) as the center.

Then the arc OB is the distance that the tortoise B has traveled, that is, the arc OB is equal to the circumference of 1/4 circle, that is, 1/4*3π meters.

Divide it by the speed of 1cm/s, and you can get the time.

The above is a personal understanding and can only be used as a reference.