@John ******
Good advice on your initial application for a DTV you only have to show a letter of proof for attending a soft power that is acceptable for the visa application. As far as I know and from a good source you do not have to show how many days, hours, weeks or months you have booked on a course. Once you have the visa and have entered Thailand for 180 days then left & re - entered I suppose it would be upto the IO., whether they can be bothered questioning how long you intend studying whatever.