Depends 100% on the carrier!
I had two SIM in Malaysia, Digi and U Mobile
DiGi expired after 6 month if not recharged, U Mobile is valid for 2 years.
It's possible to extend the DiGi's expiration date to 1 one year by paying extra 30 RM or so.
Same in Singapore: StarHab is valid 2 years, SingTel only 6 month.
When I left Singapore two weeks ago, I bought a reload voucher, so before the card expires in April, I will recharge it and can use it to the end of 2012
Sometimes, when you check the card's balance using the GSM codes (like *113# for Starhub or #107## for 3HK), it will show you the card's expiration date