Two years, on an 18v Makita, I've replaced one. I find the ability to hold a charge diminishes in about a year. You need at least two batteries, one cooking at all times, to do any serious work with a cordless drill. A construction site these days has turned into a life support station, for re-charging batteries. In fact, I think we have more cords lying around:)

But my use and yours is probably different. I may charge a battery several times a day and there are only so many re-charges available, on any battery.