What is the latest I can apply for a one-year extension of stay based on being married to a Thai citizen?

The last day you can apply for an extension is the day of your current stamp's expiration. You don't have to have specific (or minimum) days left.

This group would recommend you to go to apply several days earlier in case any additional documents required or get a hand out for the exact requirement in advance.

whilst emphasizing "meet the requirements" which goes for

@Nigel *********************

as well, just in case he forgot about that

Depends on the office that you use. They told me in Pattaya that the minimum was 15 days and the max 45.

You need your 400,000 Thai baht bank account in place two months before you make application for the one year extension Married to Thai

I would do it a week before your visa expires. Make sure all the documents are prepared and ready . I did mine last month and I got my under consideration on the same day as applying.