If the player fails the save, he believes it to be real. He'd need some in-game reason to be suspiscious and get a new try, he can't just decide to roll again. And there's already rules for getting a new save (for no action cost) if an ally points out it's an illusion, or being presented with incontrivertible proof it's fake, etc... So I don't see many situations where this would even be an issue. But if it were...I'd rule it like Move Silently, Jump, etc... It has no action on its own typically. The attempt to disbelieve is part of the action used to discern if it is real. For example, when making spot or listen checks that are NOT reactive, it is typically a move action. If he happened to have the Quick Reconniter feat, the player could examine an illusion and get a new save as a free action, I suppose (again, assuming a legitimate reason to be doing so to begin with).