Read it aloud at a comfortable easy pace. Let's say your script takes 9 minutes to read. Wall to wall narration which fills every second of the video is not especially pleasant nor effective. We're not selling Ginsu knives here. Don't risk losing your audience.
Some scenes play well without any narration.
Take a guess at how long these scenes will run and add that to the running time. Revising the script is how you turn a mediocre script into a great one - one that accomplishes your goal. The time to stop revising is when you believe the script accomplishes its goal and is as good as you can make it.