How soon can I apply for a 30-day extension after obtaining a 60-day Thai visa?

You will have to depart the country and apply for the 60 day SETV visa at a Thai consulate/Embassy…you can go anytime within the 45 day visa waiver stamp window but i would do it towards the end (last 3-5 days to maximize the 45 days)…you can go via land or by air, you can go alone or with a visa agency, totally up to you just make sure you fill out the paperwork and get an appointment at the Thai consulate/ embassy of your choice…once you return if you’re in Bangkok you can go the same day to the immigration office and ask for the 30 day extension as long as it’s still during its business hours…good luck

Some offices with 45 days left, most are 30 days but Jomtien is just 7 days left