less than 24 hrs according to the FAQ on the home page, as shown below

but I just called customer service, i was told it is 45 days.

anyway, i will buy a voucher asap since my airtime expired at the end of september.

13a. What happens if my airtime expires of my balance reaches $0?

If your airtime expires: You have less than 24 hours to refill or your SIM card number becomes void and you need to buy another SO SIM.

If your balance reaches $0 (before your airtime has expired): You have 45 days (or until your airtime expires, whichever is earlier) to refill after your balance reaches $0 or your SIM card number becomes void and you need to buy another SO SIM.

I screwed up and forgot that my account expired on Oct. 13 (balance > 0). I read the FAQ about losing my SO account and SIM so I called customer service. I was told by a nice agent that if I bought a top up right away she would be able to reactivate my account manually. I did so, called back and my phone is working without further fuss. So I don't know about the 45 day limit, but 31 days after expiry did work just fine.

Thanks for a great site!