Ninjask does have a limit, his ability speed boost raises his speed one stage. The highest stage you can go to is stage six, making his speed 400% the original, this is the stat chart below.
-6-25%
-5-29%
-4-33%
-3-40%
-2-50%
-1-66%
0-100%
+1-150%
+2-200%
+3-250%
+4-300%
+5-350%
+6-400%
Whenever you use a move like howl that raises it one stage, and when moves say harshly fell or sharply rose, then that means 2 stages up or down.