According to Bulbapedia, (credit to Amethyst for providing the link)
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.
I could not find any clear percentage in sites such as Serebii and Bulbapedia. I can, however, go off of past experience. In my couple of months replaying ORAS, I have found that acquiring stones in Super Training is a somewhat tedious process. Beating the target time does not guarantee you the best prize (in your case, a Dawn Stone), but as I stated in my comment, the best chances, corroborated by Bulbapedia’s statement.
You should also factor in the other potential prizes for your Secret Super Training Balloon (Gallade). For you, the prizes are as follows:
- Pretty Wing
- Soda Pop
- Stardust
- Hard Stone
- Dawn Stone
Hope I helped!
