Never.

As long as your husbandry is good and your snake is mite free, you shouldn't have to soak them. They will shed fine on their own. Bathing strips them of their natural body oils that act like a lubricant between the new skin and old skin during a shed. You can actually make a shed go bad if you bathe during a shed cycle.

If your snake has a bad shed, there are better options to soaking. Soaking can be stressful for ball pythons. Especially since they are a terrestrial, non-aquatic, species.
A better alternative is putting them in a warm damp pillow case and letting them move around for an hour or so. That usually gets off any stuck shed.
Or if it has only a little bit of stuck shed, you can just leave it alone, bump up your humidity, and wait for it to come off with the next shed.