Currently, unused credits in inactive accounts expire after 30 days.
From 30 October 2008 GMT, unused credits over 90 days old, will expire. New credits purchased will expire only if they are unused after 90 days. As you use or trade credits, the oldest credits will be used first. This applies to both free and paid credits.
For example - On 31 July you have a balance of $3 and use $1 in August. Next purchase of $5 is done 30 September. At 30 October (90 days later), your balance is the $5 you purchased in September. The remaining $2 from August is expired.
The above information is extracted from the website.
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