Lots of 20 baht topups(perhaps less than 20 baht is possible via Internet Banking and top up machines outside convenience shops) when you first obtain the SIM, taking it initially to a full 12 months of validity. Activating International Roaming in order to make topping up easier when abroad. Each top up adds one month, so it’s better to do your top ups in small increments. I’ve been known to do a 200 baht top up at 10 x 20 baht, adding ten months of SIM validity in the process.