I use the Flutter (although it's not the Acapella, it's really the same thing)...and I use it for 5-10 mins 2x/day. My doctor seems to think it's plenty sufficient.
For me personally, I base my time doing the Flutter until I feel clear, so some days it might be a few blows and coughs, and other days it might be a much longer session. I just do it until I feel like I'm done!