How soon can I apply for a 30-day extension after entering Thailand on a 30-day exempt entry with a US passport?

With a US passport, you can apply for a 30-day extension right after entering Thailand with a 30-day exempt entry. While some offices may prefer you to apply when you have 15 days or less remaining on your entry stamp, most allow extensions when you still have 30 days or less left. It's advisable to apply early, as there is no downside to doing so—your extension will simply add an additional 30 days to your current visa expiration.