I tend to burn through a set of CR123 batteries each week to week and a half in my Surefire carry. When the light cycles down due to lack of power to the lower setting I replace them at once. Ive been throwing the 50-70% used batteries in a box now for a couple of years since they still function fine on the lower setting. Costs me nothing and one day it might be pretty valuable to have thousands of hours worth of 5-15 lumen run time.