I suggest contacting schools directly, they might have some advice. From what I know you have to be in the country. When I contacted a school recently they said it takes 50 days (might differ from school to school). You would have to get a longer Tourist Visa at your country's Thai embassy. There might be some tricks you can do like going to Laos and getting the Thai visa there or flying in and out again but I have no experience schools might know so I would contact them directly. I went to AAA Language School, they were great at the Visa stuff (you quarterly go to the immigration but they streamline it well) and the prices were okay. Online lessons twice a week were soso, listen and repeat textbook stuff but I don't have a comparison. Best of success!