That brings up Layonara magic. As far as I've seen, using magic in Layonara is not exhausting. It doesn't seem to take a lot of effort. A caster can cast three million buffs over the course of two minutes and run off like he just got off the couch. When casters are out of or low on spells, they aren't tired; they just say, "Nope, can't do that right now."
To be honest, if I was writing a book on a Layo Quest, Sorcerers (Gandalf is "no conjurer of cheap tricks" but he has no spell book! he a sorc) would be worn out since they ARE putting effort into spells. Wizards are reading some words and making gestures.
Using magic in the NWN version is not exhausting, but that's a system thing., you just get your spells based on DnD rules and that's it. If NWN came with a mana or stamina thing, then it would of course drain from that. But it's NWN, so it doesn't.