The idp is valid for a longer period than 3 months (I can't remember how long 12 month's?), but it is only valid in Thailand for 3 months at a time, so if you are planning to retire here, it becomes invalid after 3 months, as everyone is stating.
Up to you if you want to risk it, as long as you don't crash into me!
yes, it seems a bit confusing at first, but if you get the non o in your home country first, everything in Thailand is very easy really. I have extended 3 times now and it has taken me on average about 40 minutes each time at immigration, plus a few minutes at the bank each time. Just be organised, get a folder together to hold copies of your documents and you can do it easily each year. Good luck and enjoy your life in Thailand.