For me, it worked like this
- 1st year - fun and fascinating, felt like a long holiday
-2nd year - pretty good time but missing some things about home
- 3rd year - getting tedious, I know that I prefer the UK as a long-term home, and am 100% sure I will be leaving soon
- final year - getting shite, making plans to leave, then leave
I know of other people who refer to the 4 year itch, and I fully understand it now. To stay longer than that, IMO, is only for hardcore Thaiophiles, people who hate their birth country, and people who have burned their bridges so can't face going home and starting at the bottom of the ladder again.
Thailand is a fun, bizarre place to spend a few years and see a different side of life, but as a long-term home, no thanks.
To throw away the luck of being born in the developed world, my green and pleasant land, in exchange for a poor, hot, developing country, a sleaze and peasant land, and spend my life on visas and permits at the whim of some pig-ignorant immigration official, isn't for me.