So is the max expiry date 1 year from date of purchase?

PS other than truemove which has the 90 days expiry function where i think they deduct 6 bht from the credit does DTAC have such a function?

AIS has 1 month extension plus a 30 min free call.

Are you referring simply to a SIM that can have money added to it as needed (which never expires) or a SIM that is inactive when, say, the owner leaves the country for 6 months?

Add-on packages for Happy Tourist SIM customers while stay in Thailand

DTAC 180-day Validity Extension

Extend your Tourist SIM validity for 6 months

Service fee 50 baht / time

DTAC 365-day Validity Extension

Extend your Tourist SIM validity for a year

Service fee 100 baht / time

or use the DTAC JaiDee Day Giveaway Service