http://www.truphone.com/en-US/Products/Tru-for-you/Tru-SIM-help/SIM-How-to-guide/#question931
Credit won’t expire, but there is small charge of $3 a month for unused accounts after 90 days.
Not sure what exactly this means. Maybe it's the way they encourage you to use up all funds within 90 days since credit added in your account.