Will I receive a one-year marriage visa extension stamp the same day I apply?

It might seem obvious, but don't forget to take the spouse when you go to apply for the extension and go to your intended immigration office first to get the requirements list. It varies...as I found out after recently changing from Chang Wattana to Koh Samui!

You have to go to the local immigration office at least two times more to complete your 1-year extension procedure.

You will go to your local office bringing all the required documents to apply for a 1-year extension based on a spouse to a Thai national. IF your application is accepted, you will be stamped for 3 weeks ~ 1 month as the first part of your new 1-year extension as "under consideration" on the spot. Then you will go back to the local office again on the day of the "under consideration" stamp to get another stamp for the rest of the year.

During the under-consideration period, you may or may not have immigration officers home visit or witness interviews at the local office depending on your local office. Check with them when you go to apply for a year extension.