How does the approval process work for the Non-B employment visa in Thailand after the initial 30-day stamp?

Are you talking about the first extension off the Non-B visa? It's possible you get an under consideration stamp, which says to return in 30 days. This is because the office does not have the authority to approve your paperwork and they send it to the main office. They wouldn't accept your application if they didn't think it would get approved, so just return when your stamp tells you to, and you'll get the rest of your time. They won't call you before then.