I put it the humid hide in there once I see them go blue, and leave it until they shed. Predicting a shed isn't an exact science as it's affected by how fast they grow - and that can be affected by a range of factors. Even my adults still aren't exactly predictable.
I prefer humid hides to misting. With misting, the entire environment gets damp and the snake can't escape it. With a humid hide, they have that damper environment to choose, if they need it. Normally, a Corn going through a bad shed will choose to use the hide as long as it feels secure to them.
Having said that, plenty of folks do mist and it works fine. You just have to try and figure out what works best for you and your snake.