How long is a retirement visa valid if obtained while in Thailand with a Non-O Visa?

you aren't gonna get ANY visa when you're here 😮 you HAVE a Non-O multi-entry, so when you come in you'll get stamped in for 90 days. THEN when you have 30 days or less left on that you go to the immigration office and apply for a year extension (by meeting the financial requirements, etc) for 1900baht and after you get that year extension inked in you can buy a re-entry permit.

The extension will be a year from the date your 90 day entry stamp expires and the re-entry permit will have the same expiration,.

So if you enter in Oct on a Non-O multi-entry visa, you'll get stamped in until January and that means at most offices you can apply for an extension in Dec (when you have 30 days or less left on your 90 day stamp. SOME offices allow you to do it when you have 45 days or less left so check with your office when you get here and see what they do