A variety of factors determine how long a rechargeable flashlight can hold a charge, so this spec will largely vary from model to model. Most flashlights offer different brightness modes, and running a flashlight at its maximum brightness will drain the battery faster than running it at its lowest brightness. Some flashlights can run up to 20 hours on medium mode, while others offer different run times for each setting. With age and use over time, it's likely that the battery retention won’t be the same as the first day you purchased it.