First of all, I did try a search for the information but did not get any results. Perhaps my error.

My married to a Thai visa expires on March 4. When can I appear at Hua Him immigration to apply for an extension? Can I do it the day before it expires, two weeks early, or on Feb. 4?

Thanks!

You can do it up to 30 days early.

Best do approx 28 days before the visa expires.

You will then have time should immigration require additional information/ paperwork.