90 days if the last credit you made was for at least Bz$50. Less for lower amounts. They send a text message when your credit is made and I STRONGLY advise you to keep that message on your phone. Save it to your SIM card if you can, in case your phone dies in the meantime. And if you get a paper receipt, KEEP IT somewhere safe.
I recently added Bz$100 to the existing Bz$100-odd the day before I left for an overseas trip, specifically so my credit would remain good (the entire credit expires on the expiry date of your last credit, so you can effectively extend the lifespan of older credit by adding to it). But when I returned, less than two months later, I found my phone wiped of credit. BTL insist I hadn't made a credit for more than three months, and without that message I was unable to prove otherwise. Another reason, I suspect, that they now do everything electronically - few people will trouble themselves to retain evidence of a credit so BTL can cancel "good" credit at will. It's the second time they've done it to me.