What Is the Earliest Date to Get a 30-Day Extension for a Visa Exempt in Thailand?

Earliest date to get a 30 day extension for a visa exempt? This question came up in another group - US passport holder with visa exempt, due to get an extension on 13.4. When is the earliest date one can get the stamp and still get the 30 days added to the original expiration date? I once did this a few days early and got all my days, somebody else said you can get it max 7 days before expiration. Thank you