my 30-day AIS tourist SIM is still active 12 months later.
I let it lapse after the first month. After 3 months back at home, I was able to put a few baht on it and it fired straight back up.
Now I just make sure to keep an eye on it while I'm out of the country (home) and keep at least a few baht credit on it, until I'm in Thailand. No problem whatsoever.
I think they keep the number reserved for at least 6 months, if you let the credit expire.