There's no firm estimate, but it should last for years. An off-the-cuff estimate would be 3-5, with more than that pretty common. Let's put it this way - we've been selling our D10/D100 series products for over a decade now, and we're not selling thousands of replacement batteries (ie, the ones in people's products are lasting). All rechargeable batteries lose ultimate capacity over time and charge cycles. Normally, the battery in your D10/D100 will be fairly charged, so it won't be cycled say as much as your cell phone or laptop is. The ultimate test is to do a rundown check from full: if you get enough time out of it for your liking, you're good to go. We recommend you do a rundown check yearly so that you know that the battery will be able to power your product when you need it to in the event of an electrical problem.