When you use an utterance successfully, you must wait for the universe to stop echoing with power before using the same utterance again (you may, however, use a completely different utterance). The cooldown time is equal to the 1 + the level of the utterance. ''For every 5 points you exceed the truespeak DC for your utterance, the cooldown time is reduced by 1 round, to a minimum of 1 round'' (so it can be used again at the start of your next every other turn). Thus, you can use an utterance on a [[SRD:Dark Naga|dark naga]] (CR 8, so DC 19), and if you get a result of a 30, you can reduce the cost by 2 rounds (minimum 1). None of this "unable to cast for the next 24 hours" business.