When can I apply for a Non-O visa extension at Nakhon Ratchasima immigration office?

Hi everyone, I will be entering Thailand on a 90 day Non O on 6 July. I intend applying for a 12 month extension based on marriage to a Thai national as soon as possible after that. I am married already and have had a bank account in Thailand for many years. The 400,000 has been in that account for over 6 months and all the other documents are in place except for an updated Kor Ror 2 and that's easy enough to obtain My local immigration office is Nakhon Ratchasima (Korat). Does anyone know when I can apply for the extension at that office? I've heard not until the last 15 days of the Non O and 45 days into the Non O. Can't get them to answer the phone to find out.