What is the process and timeline for extending a 90-day non-O marriage visa to a one-year extension in Thailand?

You would like to apply for an “annually extended stay permit” from out your Non-Imm-O visa.

The “one year extension of stay permit based on being married to a Thai wife”

Immigration will simply call it “visa extension”, but actually they don’t extend a visa, they are extending a stay permit.

You can apply for a “1-year extension of stay based on marriage with a Thai wife” 30 days (45 days on some immigrations such as Bangkok and Chiang Mai) before the end of the 90-day stay permit stamped and approved upon entry .

To do this, your marriage must be recognized and registered in Thailand. Because immigration will want you to present a freshly printed from the Amphur Thai marriage certificate.

In addition to other documents to be provided, financial evidence must be provided:

Either

You have a Thai bank account in your sole name and a minimum of 400,000 Baht have seasoned for 2 months on the day of application,

or

You have legalized proof of Embassy in Bangkok/or an honorary consul of a regular monthly income of at least 40,000 Baht (an income affidavit)

Some embassies don’t issue affidavit of income any more. In this case, Immigration will ask you for a bank statement proof of 12 months consecutive money transfers coming in from abroad over a minimum of 40,000.- THB

The combination method as with the “retirement extension”, a mix out of a deposit plus income, is not possible for an extension based on being married to a Thai woman.

If you meet all the requirements, immigration will stamp your passport or clip a paper into it, stating “application under consideration, come back at . . . (and a date, in most cases 4 weeks away)”

Within a month, immigration will visit you and your wife at home home and ask witnesses whether you are really married and live there.

As soon as the “under consideration” period has expired, you can pick up the passport from immigration with the completed and stamped yearly extension of stay permit.

Once you receive the visa, after about 60 days you can apply for the 1 year extension. Get the list of requirements from your immigration office so there's no surprises and then gather everything and apply. You'll likely receive another "under consideration" stamp.