Batteries are rated in "mAh". This mAh rating is an average of what you should expect from the battery, but results will vary due to how high of voltage you use and how old the battery is. An older battery will tend to not carry a charge as long as a new one. And there will be some slight variances from one battery to the next.
Using a low resistance atomizer device, as well as those with dual coils, will drain a battery faster than a higher resistance atomizer device or single coils.
100 mAh is equal to 1 hour battery usage between charges. So theoretically, a 650 mAh battery should last 6.5 hours in normal use.