Every night, unfortunately.
There's really not a lot that you'll have direct control over, when I got mine I got what my provider gave me. I ended up with a REMStar Pro BiPAP (two levels of pressure, one for inhale & one for exhale).
You can't just go get one, they're prescription only. You'll have to have at least one night in a sleep clinic, potentially more. My night consisted of a couple hours to determine that I did indeed need one, then the remainder of the night on an autopap to determine the pressure at which the apneas stopped (too much pressure causes central apneas).
Once you get used to it it's no big deal, in fact it doesn't quite feel right sleeping without it.
As for results? I've had mine for about six months, but I think it's been useless. Well, not entirely useless but not nearly as effective as it could/should have been. A couple weeks ago my brother informed me that I had been sleeping with my mouth open & leaking all the pressure out. The g/f confirmed this, so who knows how effective it's been. I purchased a full face mask but got the wrong size. Even with the wrong size I think it's better than the nose only mask, hopefully once I get the right size I can truly get the benefits.
I do know that I felt unusually good after my 1/2 night in the sleep clinic, I have yet to experience that at home. I was using a properly sized full face mask then.
Viper GTS