That is one of my questions also! It is difficult to know when it is enough and always question if I'm really cleared out.
My time spent doing airway clearance is way too long. I'm looking into getting that Pari eFlow to help cut the time nebulizing anyway. It's a lot of money so convincing myself to buy it is difficult, but when I think of all the time slipping away from me it gives weight to the pro side.