What is the longest duration I can stay in Thailand, and how can I extend my stay?

You can spend the rest of your life here dude! 🕶 Where there's a will, there's a way!

I got here from uk about 10 days ago. Im jabbed but no 1 questioned didn't ask for insurance just walked through 3 airports. Got 45 days on arrival and im going to get q simple 30 day extentioned that looks simple and stay another month. In May I could may e do a border bounce

There is always the 5-20 year elite visa for a price of ฿500k to ฿1 million baht

Thailand has a very complicated visa process.

Get a METV, you'll need to do some border runs but you can stay up to 270 days total if you time it perfectly. Probably the best / cheapest option to stay long term.

this is a hard question to answers ill start at the beginning...1 you arrive and get a 45 day Visa free entrance stamp then you extend it for 1,900 baht at immigration giving you 75 days... 2 ... you go to a Thai consulate/Embassy in your country and buy a 60 day SETV that you extend for another 30 days In country giving you 90 days....3 you buy the 6 month METV and with strategic border bounces it gives you 9 months of stay in Thailand...4... you arrive receive the 45 days then buy the 30 days then you do a border bounce to give you another 30 days that brings you to 105 days... ..5... you arrive get the 45 days extend 30 days do a border bounce get another 30 days (75 days so far) you go to a Thai embassy/consulate get a 60days SETV with a 30 day extension that gives you 165 days.... 6...you fly out/In every 45 days until an Immigration officer says DENIED... I hope this covers what you are looking for, note you get 2 land border crossing per calendar year (Jan-31 DEC) if you buy a visa and do a land border crossing it does not count against your 2 land border crossings per year, fly Out/In no official amount of times you can arrive just be aware you will run into that one over zealous Immigration officer that will say otherwise... the 45 days of Visa free waiver will expire at the end of March unless its extended, from what we learned during COVID is that its never over until the last day...

The longest you can stay is theoretically until you pass away. How you achieve staying that long depends on a few factors. Your current age would be one. Whether you intend to marry a Thai or have half Thai kids or work would be others.