Those of you (and I know you are many) interested in a career in voiceover might want to read Ken Levine on the success of Mark Elliott. Mark is in a very tiny group of folks in this area who work constantly because, among other reasons, he's really good.
I hired Elliott once when I produced a prime-time kids' special for CBS. Two rules were then in force at the network…and if they still are, I don't think they're as strict as they were then. One was that on a show for younger audiences, there had to be a clear separation of program content and commercial content. When you took a break for ads, a voice had to say something like, "We'll return to our show right after these commercial messages." And then when you came back, that voice had to say, "And now, we return to our program" or words to that effect. It was to make sure every kid knew when the show stopped and the commercial began.
The other rule was that the voiceover had to be by what they called a "Network Quality Voice." This is sometimes a concern at some networks. Someone believes that a voiceover that talks about their shows or says, "Stay tuned for [whatever]" represents the network and is part of their image. So they don't want just any old voice in there. They want someone whose voice reflects the branding of the network. I was handed a list of the currently-approved announcers and told I had to hire one of them. There were six or seven names on the list and I picked Mark Elliott.
He came into the studio where we were editing the show. We put him in the booth and had him read, "We'll return to our show right after these commercial messages." Then we had him read, "And now, we return to our program." Then he signed the time sheet and left. Total time he was there: About three minutes. He was such a pro we only needed one take of each line but we recorded them three times, just for protection.
One of the crew guys grumbled a bit and said, "You paid him all that money for three minutes work?" Before I could say anything, the director said, "No, we pay him all that money because it only takes him three minutes."