It depends with school that help you submit the documents? To get extension you need to find a school to register and submit documents, you should have at least 30 more day to do documents
sorry for that for give me, I just try to help and thank to correct my spelling, I am still learning since I am not a English language speaking and thank you for this
2.Open bank deposit 800,000 ( it have to be out side from Thailand transfer
Keep there for 2 month)
3. Apply non O 90 day about 20 day them you will get stamp
4. get cover letter 800,000 again 45 day before visa expires now you can apply for non retirement 12 month and you will get stamp on the day if everything approved
So very much you have to put 4-5 month 800,000 in there
To apply they require 15 day before your current visa expires