Frankly, I don't see why it needs any prerequisites. Essentially, giving you the power to end your own spell-effects before you've gotten the full benefit of them could easily fall under the category of metamagics that don't need prerequisites or level adjustment. I mean, I can't think of any instance where you might willingly end a spell effect early. Unless maybe you're using Symbol of Pain as a torture device or something.... Or you could end debilitating spells early in the event an enemy surrenders.