Assuming you mean that you meant that each side was 15 centimeters long, then the worm crawled 60 centimeters total. This is because a square has the same length on all 4 sides, and if one side is 15 centimeters, and there are 4 sides, 15*4 is 60.
However, if the side was 15 <some other unit here>, then the answer above is wrong. Technically, you didn't give enough information to solve this.