I'd do this by testing the touch position against the position of the last prefab and if it meets the threshhold - then create a new prefab.

Here is some untested C# code (my syntax checking isn't nearly as good as a compilers):

//last position where we drew the dot
Vector2 lastDotPos;
public float dotDropDist = 10; //min distance between dots

void Update () {
    foreach (Touch touch in Input.touches) {
       //if we start touching
       if (touch.phase == TouchPhase.Began) {
         Instantiate(prefabDot, new Vector3(touch.x, touch.y, 0, Quaternion.identity)
         lastDotPos = touch.position;
         break; //ignore the rest of the touches if any
       } else if (touch.phase == TouchPhase.Move) {
         //get the total pixels moved
         float distance = Mathf.Abs(touch.position.y - lastDotPos.y) + Mathf.Abs(touch.position.y - lastDotPos.y)
         //if we meet the threshold the create a new dot and reset our last dot position.
         if (distance > dotDropDist) {
          Instantiate(prefabDot, new Vector3(touch.x, touch.y, 0, Quaternion.identity)
          lastDotPos = touch.position;
         }
         break; //ignore the rest of the touches if any
       }
    }
}