I've been staying in Thailand for about a year now on SETV's and Exemptions. I will extend my current SETV and fly back to Europe at the end of December.

 

Earlier this year Immigration almost send me back to Malaysia when I flew into Don Muang airport, so I already know it's quite risky to fly once you have stayed in Thailand for a certain amount of time.

How long do I have to stay outside of Thailand before it's save to fly into Thailand again, without risking to be sent back?

1 hour ago, khaoniao said:

How long do I have to stay outside of Thailand before it's save to fly into Thailand again, without risking to be sent back?

That’s an impossible question for anyone to answer as there are not set rules. 
 

The longer the better, but ultimately it’s down to the IO every time you enter and their opinion on the day.

How long you must stay away is a combination of luck and where you enter. If desperate to return, get a visa, fly to Kuala Lumpur, then

• fly to Vientiane and enter at the Friendship Bridge to Nong Khai; or
• take a bus to Hat Yai via Sadao.

Each airport and land crossing is now its own fiefdom, allowed to create its own rules, with little or no oversight from above. You need to know which entry points are still following the Immigration Act as written, and which have gone rogue.