How much fatigue does Armor cause a demon? 1 point? 1 point per time it actually has affect in play? 1 point per round?
Well, there have been two equally viable/'official' answers to this one.
- All the abilities fatigue the demon one each use, but recognize that Sorcerer isn't based on "physics," and so it's about seeing the ability happen, not about whether it's "turned on." So if that demon were not hit in that round, the Armor wouldn't cost it any energy. 
- Armor is like Fast; it's turned on once as an action at the start of a combat (or prior to the combat, given a demon with some brains and/or prep), and then continues to work without the need for further actions, and it all counts as one use. 
So... the answer is "pick one and make it your official answer."
See also: Mundane Armor