That depends on the quality of the battery and a few other things. Generally, a tiny coin battery will last about 70-100 hours when set to a timer. That means about two or three weeks if they're working every single day. There are ways to make the candles last longer- first, make sure you're choosing high-quality batteries as replacements. Second instead of using the timer turn the candles off at the bottom when you don't plan to use them for a while. The timer is very convenient- but the candles need to use a little power all the time they are waiting for a remote signal.