If a player wants to turn on a power out of combat (e.g., be hidden or whatever that power is) during a discussion, whats the best/correct way to adjudicate that? I don't want to force them to keep rolling to sustain it (that makes sense in a combat/emergency situation).

If there's no reason to roll, don't roll. That's the general rule of thumb. If nothing interesting could happen, then it's not worthy of any rolling.

So just let him sustain it, but remember that it's taking concentration to do so. If he wants to do other stuff that might distract him, force him to roll to keep sustaining.

That's what I'd do, anyway.

In DH, the sustain roll is made once per 10 rounds of combat. A round is approximately 5 seconds of in-game time, so the roll should be made about once a minute.

Obviously, it'd be a huge pain, so I'd reserve that knowledge for special cases, like when the failure could have really fun (or horrifying, depending on how you look at it) consequences, or when the player is getting too smug about using his psychic powers.

If someone is abusing their psychic powers I guess you could allow them to do it, then suddenly drop a willpower test on them. If they fail they get D5 corruption points.

You explain to them that this isn't Harry Potter, you cant just use your powers for every day occurrences and expect nothing bad to happen. They are transmuting immaterial power that is the essence of demons.

The warp cant be taken lightly!