I don't think there's any correct answer to this since I didn't find any percentage of finding one by doing the training regiments, however, You can get them another way other than just doing the super training regiments.
>The player may earn additional training bags by participating in Super-Training Regimens, or by "punching" (tapping) repeatedly on the empty training bag on this screen. Pokémon will also hit the training bag automatically, once per minute, occasionally finding training bags on their own.
Source: Bulbapedia
