Keep in mind when considering shifts needed that not all targets are 'Main NPCs'. If you summon a generic demon, he's likely to count as a grunt (no consequence) or a supporting NPC (moderate consequence). "Taking them out" is much easier. (Of course, they will also be of less value in a fight.)
As to Fate points to buy out of compels ... I think it's guaranteed that they have at least one, since the fact that they were summoned against their will would surely count as a compel against their High Concept.
Finally, if the Demon conceeds, then he doesn't take any consequences at all, nor any stress boxes. I suppose he could, if it suited him, mark off consequences (and get extra Fate points for the concession), though that would also possible hinder his attempt to break free.
As to duration ... I'm thinking that you'd probably treat the binding as giving you the ability (for the duration of the Thaumaturgy used for the binding) to compel the demon's High Concept to force him to do things. Of course, this means that you are spending a Fate point each time you toss out new orders, and that the demon is getting those Fate points. Eventually the demon is going to refuse a compel (by spending one of the Fate points you've given him). Then you'll either up the ante and he'll back down, or you'll find that you can't afford the compel. And at some point, you'll have given him enough Fate that he has more than you, and can break free at his will.