When is the Soonest I Can Apply for a Non O Extension in Sukhothai?

The O/P is using the Sukothai immigration office and as he said they allow 30 days or less for extensions, so at least he knows for sure when he can go

We wish him the best of luck with his upcoming extension (y)

Visit immigration well before and ask the question, it is pretty easy even here.

depending on your citizenship (you seem to have a British passport) on the day of application to the 1-year extension, you will need a minimum of 400,000 THB sitting in a Thai bank account in your sole name, for a minimum of 2 months. And on Immigration, you need to prove this with the letter statement of the bank and your updated on the same day bankbook

Typically it's when you have 30 days or less left on your permission to stay stamp, but in some of the larger cities it's up to 45 days. You'll have to contact them and find out what their particular policy is. You should always contact your local immigration office well ahead of your extension date to find out what the current requirements are for your particular type of visa extension, marriage extensions usually have a lot of requirements.

You are now able to make application for the one year extension of stay as Married to Thai at your local Thai Immigration office.

You should have gone by now and asked them for the handout list of requirements then you would know