How many days before my exempt visa expires should I apply for a 30-day extension in Thailand?

You should apply for a 30-day extension of your exempt visa when you have 30 days or less remaining on your current stamp. Some immigration offices, like those in Jomtien and Samui, recommend doing this when you have 2 weeks or less. It's important to check the specific office you plan to use, as procedures may vary. When applying, ensure you have a copy of the TM30 form from your hotel, even if it might not be requested.