When should I apply for a yearly visa extension based on marriage to a Thai spouse in Thailand?

You can apply for your yearly visa extension based on being married to a Thai citizen when you have 30 days or less remaining on your current visa. Specifically, for your visa expiring on November 26th, you can apply starting October 27th. While you have the option to wait until the last day to apply, it is advisable to submit your application earlier to avoid potential issues.