Can I extend my 60-day visa exemption to 90 days now, or should I wait until January?

Thank you, but this post is not going to collect each office's "max remaining days" policy...

Just so everyone is clear, you would be hard pressed to find WORSE advice on entry/visa/extensions to thailand than you get asking any of the AI programs out there 😮

It's because so much bad information is out there, when it data mines it uses the bad info instead (because it's been repeated so often)

Be that as it may,

In bangkok you can apply for a 30 day extension on your 60 day visa exempt entry stamp when you have 45 days or less left on your current stamp. To get a 30 day extension on a 60 day visa exempt entry you go to IT Square Laksi Plaza, you go in person. You need a TM30 showing you're registered as staying at a place in Bangkok

No need to run your stamp down, you don't lose any time applying early, they add the 30 days to your current stamp expiration date