Come Up Short

How long should your script be?

No, this is not a trick question. Yes, in reality, a 30 should be 30 and a 60 should be 60. And yes, in reality, writers write 36s and 68s. But the real answer is “less than it needs to be.”
The problem is the client wants to say more than what you can say in the course of the spot. Believe me, I feel your pain. But a spot is almost always better when the voice actor gets the opportunity to present his best performance and that almost always takes longer than what the copywriter expected. So I recommend writing 1-3 seconds under the time limit. :28 for a 30 and :58 for a 60, for example.
If getting the best performance isn't incentive enough, consider the fact that it makes you look like you know what you’re doing when you’re not trying to cut 6 seconds. Reading at a normal pace also makes it easier for the listener to understand the message.
Again, read it to yourself with a stop watch. Aloud. Multiple times. And “come up short” on the length.