When should I apply for a 30-day visa extension for my tourist visa in Bangkok?

What stamp is in your Passport:

« TR–60 » or « ผ.60–ม.17 »

will determine where you go in Bangkok to get a Tourist Extension of Stay

Any « TR–60 » Entry Stamp would also mean you should already have a Hard Copy Printout of your Thai E–Visa PDF File from the Thai Embassy/Thai Consulate that you paid ฿1000 Thai Baht (in local currency equivalence) to for a Single Entry Tourist E–Visa (SETV)

If you do indeed have a Tourist Visa then the expiry date does not matter as that visa is now used and finished. What matters is your permitted to stay date that is stamped in your passport as this is the date that you will be extending.

If you bought a real tourist visa online from a thai consulate before you came to thailand and got the 60 day stamp when you entered (TR60) you DO NOT go to IT Square Laksi Plaza to get an extension.. You go to the main immigration office at the government complex (Building B, 2nd floor, Section J) on Chaengwattana Road,

Now on the other hand if you are just showed up here and got stamped in free stamp (visa exempt) for 60 days (ผ.60-ม.17) you DO go to IT Square Laksi Plaza (3rd floor, Section K)

Either way you can apply for a new extension when you have 45 days or less left on your current entry stamp. You do not lose any time applying early as the 30 day extension is added to the expiration date you have now.

So the real question to you IS

did you buy a tourist visa before you came here OR did you just show up and get in free stamp ??