How far in advance can you apply for a retirement visa extension in Thailand?

Korat 45 days

You cannot "renew" your visa. You can apply for a 1-year extension based on retirement.

It depends on the immigration office to which you apply for an extension. Most allow extension applications when you have 30 days or less with your current stamp. Some accept applications with 45 days or less remaining. There are one or two offices that accept extensions only within the last week of your stamp.

Please check with your local immigration office for their policy.