I heard when the battery is about to die, there's a symbol for it that'll appear on the screen. I assume that'd be fair warning to go ahead and get a new one before it's totally shot.

My token is so old, I got it back during the FFXI moogle-bag promo when they first started releasing the tokens. STILL have not seen that 'low battery' symbol. So if you're using the one we got from the CE, you're probably good to go for a long long time still.