When is the last date to apply for a 12-month extension of a NON-O visa in Hua Hin?

The LATEST date you could apply for your year extension is the date that 90 day Non-O visa stamp expires (12 Dec 2020).

The EARLIEST date you could apply in Hua Hin would be 30 days before that date (any time after Nov 12th).

Some offices allow you to do it when you have 45 days or less left on your stamp BUT I believe in Hua Hin the earliest you can do it is when you have 30 days or less left.