I don't know if this is still current, but I found *9344 for Truemove and *500*9# (expensive compared to the others) for AIS.
I manage quite a few SIMs, both my own loaner pool for visitors, and for some FTers, so I just do the 12 baht shuffle each month for ~ 12 SIMs. As such I haven't stayed up to speed on the operator-direct methods. I did just try TrueMove H/*9344 (that's an IVR number) and it did not work (that promo is called
"Add Instant" , not sure why it didn't work for my TrueMove H SIM? . The AIS code actually changes your calling promotion, but you do get 30 min. of calling over 5 days as part of the 30 baht - although still not sure if the "Mao Mao" thing is still active.
If long term expiry is important the I'd recommend getting a standard SIM and doing twelve separate, individual 12 baht top-ups before you depart.