Like 50%, 30%, 10%, etc.
(before someone say, I already looked at the look alike questions and there is not that question there yet)
According to Bulbapedia:
In Secret Super Training, if the player achieves the target time, they will also receive an item as a reward, in addition to a training bag. The more the target time is beaten by, the more likely the player is to receive a rare item.
It appears that there is no fixed percentage. It should also be noted that you can only get a Water Stone in The Water Stone Cup Begins! or The Battle for the Best! (any game).
