It all depends on the location In which you apply for it…in bkk you can go the day you land as long as you will make it there during their work hours M-F…in other locations around Thailand some immigration offices won’t do it until 10 days to expiration…but every place it still costs ฿1,900 baht
as close to your departure date as possible it takes around 3 business days to process thx visa minus your countries holidays and Thai holidays so keep track of that…yes you can get an additional 3 months on the visa but not in country you will have to do a strategic border Bounce before it expires…you also can’t stay in country longer than 90 days at a time so you will have to do 2 strategic border bounces either by air or Land….FYI if you do land border bounces with the visa it does not count against your 2 land border bounces per calendar year…enjoy