When does the extension of a retirement visa in Thailand start after applying for renewal?

in reality you don't "renew" anything, you apply for a NEW yearly extension every year, year after year. Most offices let you apply for a new extension when you have 30 days or less left on your current stamp (so if you have a stamp expiring the 27th of June you can apply any time after the 28th of May)

You don't lose any time going early, they ADD the new yearly extension to your current stamp expiration date 🙂