The words at the end of the text say this:
(If Lucario is no longer your active pokemon, this effect ends.)
Basically what it means is that Stance will remain in effect for the duration unless you bench him. If you activate it, it stays on until the end of your opponents next turn, so they can't do anything to him attack wise, Cess Crystal also won't shut it down because it is "In effect.", thats why he's usually Leveled up, Stance is activated, and then a Cess Crystal gets placed on him, also because most decks Lucario is in anymore don't have pokemon powers or bodies.
So if he's benched, Stance goes away. Think of it this way, if you took a stance in martial arts, you would remain in that stance until you tried to escape, no? But it also lasts only one turn and you can only Stance once (upon leveling)
