I believed it to be 2 months too as in all my other years but my Thai wife tells me its changed to 3 months and her information come from Immigration in our province.
so is 1 year extension different to renewing marriage visa ( non immigrant O ) 4. Renewing: When renewing your Thai Non-Immigrant O Marriage Visa, you must have the necessary funds cleared in your Thai bank account at least three months before your renewal application.