Like most battery operated things - it depends, quite simply, on how much you use it. Unlike a flashlight, which you could turn on and test to see how long it lasts, LeCigs are "On-Demand" products - which means the battery is only being used when you are using (vaping) it.
So, if you do like some of us and chain-vape away - cartomizer never far from your lips - yeah, your battery won't last quite a full day. BUT, if you vape 'responsibly' and use your LeCig the way you would use analog cigarettes (taking breaks at work or on lunch, etc.) then even for a pack-a-day smoker our standard LeCig D1 Battery should get about 6-8 hours of use.