How long is an income affidavit valid for when obtained from the US Consulate Office in Chiang Mai?

Ah, this is one of those "local option" questions.  Most report that their offices accept letters that are six months old, some reports are as short as one month, some reports up to one year.  I would think the Chiang Mai office will understand about the U.S. Consulate being closed for a time and will accept a letter you obtain now for an extension in, say, November.

 

Did you know you can apply for a visa extension up to 45 days in advance in Chiang Mai province and not "lose days".  They date your new 12 month extension from the date of expiration of your previous extension.