Emily does it twice a day when well and 3-4x/day when sick. When she's sick she also increases the amount that she uses. This is easy to accomplish since she uses the Ventolin inhaler; she goes from 2 puffs when well to 3-4 puffs. This was after we established it doesn't affect her heartrate or cause any other adverse effects.
We do see that it makes a difference for her when she's sick and I know she can tell too since she will bring me the inhaler and ask to use it. It helps the chest pain, trouble breathing, shortness of breath and some of the cough she gets when ill. It's not enough on it's own when she's sick though (she needs the Pulmacort and sometimes Prednisone too- and I think any of the meds would be pretty limited in effectiveness without the extra vest sessions that go along with them.)
She doesn't have blood sugar issues to worry about (she's only 8).