Its the exact same token thats used for WoW (Digipass Go 6). I've had mine since Blizzard started using them, and I havent once heard anyone complain that the battery died in theirs yet. So to answer your question, i'm sure it varies to some degree, but we're still talking years of battery life. The good news is that unlike physically losing it, even if its dead customer support can verify that it's really you and not some scammer trying to social engineer his way into your account with the serial number of the device.

Anyway, it's long enough that it's not worth worrying about until it's been four or five years, then you might want to start thinking about preemptively buying a new one and switching it over to avoid the potential of a future headache.