Battery life is based on: the chemistry of the battery, time, temperature, the number of cycles, the number of deep cycles, how fast it is charged, and how fast it is discharged.
You can not base battery life on only one thing. As an example, if a battery last 300,000 miles in a New York taxi but that is only over 5 years, you can not expect the battery in that same model of vehicle to last 300,000 miles if it is only driven 10,000 miles per year.
BTW, with hybrids achieving savings is all about: how many miles you drive each year, the number of gallons of fuel you save multiplied by the price of that fuel, the extra cost of the hybrid system, the number of years you plan to keep the vehicle, and the replacement cost of the hybrid battery if you keep the vehicle too long.
The number of miles you drive each year is one of the biggest influences on how much you save or do not save.
Check out "fueleconomy.gov" click on "advanced cars & fuels" click on "can a hybrid can save me money" click on "select a hybrid" at the bottom of the list click on "View All (table)" and check out the comparison of hybrids vs their conventional counterparts paying attention to the "years to payback" on the right side. Then do some searches on the cost to replace the battery and the life expectancy.
The bottom line is that if you drive a really high number of miles each year a hybrid can be a great deal, but if you do not drive a high number of miles each year a hybrid can be a money pit.