I made a mistake and went to my embassy end of November for this, too early. Can I still use it end of January? Extension of O visa as married to a Thai.

According to a recent previous thread here it should be valid for six months. However, it's best to check directly with the immigration office that you are going to use as there have been some postings that some offices will not accept letters older than 30 days!

The current rule is 6 months early. But suggest you confirm it with your immigration office.

The current rule is 6 months early. But suggest you confirm it with your immigration office.

At Jomtien I just used an embassy affidavit that was 3 months old with no problem. I don't recall reading any reports of anyone being told the embassy letter they used was too old ... but of course anything is possible.

And I have used 2-month-old Embassy letters (as per the OP) for my last 2 retirement extensions at Maptaput with no problem!