Sometimes, like other voices, I receive scripts from individuals who don't need a voice over very often. I always try to bring things up to speed by communicating that when timing out your copy, it's important to read the copy out loud- not just with your eyes silently.
I received a particularly challenging 30 second TV spot from a client where they wanted high energy and fast paced. Even at lightning speed it was coming in at around 35 seconds. The client said that when so and so at the agency read it, they had no trouble timing it out at 30 seconds. I reminded him that the copy needs to be read out loud to get an accurate timing. A voice talent will also be inflecting and coloring the words, and that adds a bit of time too. The client realized that some copy needed to be cut, and all was well. As a voice talent, we can shave a few seconds off simply by voicing certain sections of the script at a faster pace and marginally slower for key copy points, like an address or company name.
For corporate video narrations, here's a quick tip to find out how long the voice over is going to be -not the entire run time of the video. Most corporate narrations are voiced at a rate of 150 words per minute. Simply do a word count of the voice over part in Word under "Tools", divide by 150, and you have your estimate of the length of the VO. So, for example, if the word count is 784 words, that divided by 150 is just over five minutes. Pretty basic stuff, but useful.
While we're talking about timing, I voice medical narrations now and then. These can require a snail's pace when being voiced, as the content can be highly technical, and the viewer (learner) needs time to digest what's being said. Sometimes the pre- recording direction I'm given is "It needs to be slow." One person's slow may be another one's medium. That's where I find voicing a paragraph or two beforehand and sending the audio to confirm with the producer to be very helpful. Works like a charm.