Phil, whereabouts are you? Reason I ask, it may be best for you to hear it yourself. If passing by, don't mind a quick stopover.
It is a small whisper of a sound and it does not bother at all. There are times, when, say, another vehicle is parked right next to you and the sound bounces off, or when it is really quiet and you can hear the compressor.
How long - depends. When I start the vehicle, maybe about 5 seconds - if it needs to adjust.
If I have employed auto level, then can take up to 10 to 15 seconds for it to build enough pressure to level.
It does not run all the time or a lot.
Also, with the soundproofing of the Cali, you hardly hear it inside. Outside, the plastic panels underneath help reduce the sound.
The THQ guys had done it on a standard transporter and that will also make a difference, and from what I remember, they did not fit VBAir.