How long before my visa expires should I apply for an extension in Thailand?

Less than a month worked for me in huahin

Gone are the days of the border run,3 months ,

Depends where you are and the type of visa you are on

No more the 7 days.

Go 3 to leave some room in case your paperwork is rejected and you need time to sort it out.. don’t want to overstay

Most Visas can be extended 30 days before the stamp runs out, 45 days at some Immigration Offices

If going to Jomtien office, it's 1 week. Every office is different.

You cannot extend a visa. You can extend your stay on the basis of a visa or on a visa exempt. It depends on the kind of visa you have and on the rules of the Immigration where you apply for it. Every hotel is obliged to issue you with a residence certificate (TM30). When you leave the hotel, your residence ends. If you have the TM 30 certificate, then go to the nearest immigration office. They will tell you whether you are too early or not. You still haven't written where you want to apply for an extension (30 days). I also don't know all the different deadlines of all 64 or more immigration offices in Thailand.