Service providers here have greatly reduced the validity periods associated with different value top-ups. So a 250 baht top-up might get you 30 days. AIS/One-2-Call did have some graduated/accelerated top-up schemes so over 10+ top-ups, the associated validity accelerated,a nd if you topped up before the validity expired then you eventually built up a validity whereby any top up value pushed the validity out to 365 days. I was a One-2-Call customer for 8+ years.

There are some means to extend validity; DTAC/Happy have a scheme where you can purchase 30, 90 or 180 days for 2, 6, 18 baht with a code ( *113*nn#, where nn= 30, 90 or 180 days), but you may need to be a customer for at least 90 days before you can use this process. AIS/One-2-Call also have a similar service, but just 30 days for 30 baht - not sure if you need to be a customer for some period or if you can do this more than once?

There are also ways to transfer balance and validity from one number to another.

There are also requirements which require some usage every 60 - 90 days, and these are reinforced sporadically as the pool of available numbers is shrinking rapidly and service providers need to recycle numbers more frequently.

The regulatory authority here, the NBTC, has been making noise for a year or so re: abolishing any validity/expiration, and an accord may have been reached whereby any top up gives a minimum of 30 days, but the usage requirement will be enforced. So you may be able to do do 12 top-ups at 10 baht each to get one year? YMMV.

You could also check with your service provider at their shop at SBIA/BKK on departure to inquire about extended validity.

I am currently on a post-paid contract with DTAC so am no longer impacted.