This is easy to figure out because you are already vaping the dripper at the correct resistance and can compare to that. Consider them 2000 mAh. If you are currently using 1500 mAh batteries, 2000/1500 = 1.33. The VTC4 will last about 1.33 times as long. The only way this will be way off would be if your current batteries had high internal resistance due to age or being low performance batteries. Then you would see more time out of the VTC4s.