I suppose that depends how much you play, Dave. I would replace it at least once a year, but more often if I were playing heavily. It's only a few screws to get to the battery, so I would check it periodically, maybe every 2-3 months, until I had a handle on the drain pattern.
Another thought is that the best performance of your electronics will come from a fully charged battery. If you are going into the studio or about to have a critical performance, then put a new battery in.
Remember that the battery drains anytime the cable is plugged in. Most of us know that, but it's always worth mentioning when the topic arises.