Yes, Out Loud
There is only one way to check the length and sound of your copy. You absolutely must read it out loud. Yes, out loud.
It can feel a bit odd to read out loud to yourself in your office or cubicle. But it sure beats the feeling you get when the first take in the studio runs 72 seconds, especially if four departments, including Legal, have approved your script.
Please don't take this to mean that your talent will be upset if you have to stop after two takes to chop out a couple dozen words. If your talent gets upset about anything other than true rudeness, get different talent. It just seems a shame to spend five minutes hacking away at a script that probably took hours to craft. If you want to hear how your beautiful prose sounds on the air, don't set yourself up for a butchering in the studio. Read it out loud.
Reading copy out loud also helps avoid problems that aren't obvious on the printed page. These include those long drawn out print-ad-type sentences that never end because there's no place to breathe and are difficult for the listener to understand, anyway. There are awkward combinations like "Fresh Step" and "safe and effective" (which can sound like "safe and defective"). There are also the gaffes that you just couldn't commit without couldn't make unless you're using a word processor and you forget to delete what you replace. Read it out loud, or better yet, get someone else to read it to you.
Whispering, mumbling, or just imagining the copy with a stopwatch won't cut it. Read it out loud before you get to the studio and save yourself a lot of artistic heartache and maybe some money, too.