The usual method is to determine the sleep current consumption, determine the awake current consumption, calculate an average consumption based on the sleep / awake ratio, and then use the average to estimate battery life.

But, I wrote that to make a point to Jack. For a Christmas gift, turning off the BOD is irrelevant. Any average current consumption between 0.1 uA and 2.776 mA is going to get the gadget to the five year mark; long after Christmas is over and the children have lost interest.