The main thing is to stay ahead of the mucus if you need to. My second nebulizing early evening does not produce much but it does produce some, so for me it is worth doing. My first nebulizing of the day done around 11 am produces quite a lot of sputum. Doing a second clearance the evening before reduces that "burden of mucus" the next morning. It helps me to stay ahead of it. But once a day may be enough for those who produce so little sputum or produce none the second time. I would talk it over with your pulmo. I used to do once a day but my Dr switched me to twice when i told him how much i was producing.