When and where should I apply to renew a non-immigrant B visa in Bangkok, and how long before its expiration should I apply?

To renew a non-immigrant B visa in Bangkok, you should apply for an extension rather than a renewal as visas cannot be renewed. It is typically recommended to apply for the extension about one month before the expiration date of your current visa. Your employer will need to assist with the preparation of the necessary documents for the extension process.