Thought this post by Porker on the chat board was worthy of inclusion here as a good archive of information.

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Posted by Porker on June 28, 2008 - 18:04 pm

They're supposed to expire 6 months after your load does, from what I understand. But you can re-load yourself via the internet via chikka.com if you are ever away for an extended period of time. You need to deposit a minimum of 10 bucks at a time via PayPal, but can use that money in increments as small as 1 USD, which gets you p30 worth of load on Globe or Smart, which expires within 3 days.

BTW, using chikka to send load is the easiest solution to another potential problem, which is falling below p100 when you use Smart and activate "roam on" when you leave the country. If you fall below a p100 balance when you come back to the PI, your phone cannot be re-activated by buying load, only by heading to the closest Smart office (any mall...). But re-loading via Chikka's "i-send" works like a charm.

Bottom line -- a dollar every 6 months will extend your SIM card indefinitely.