How Long Do Hybrid Car Batteries Stay Efficient?

Hi. I was looking into getting a hybrid although I'm not too sure about getting these…complicated machines. I'm working towards my undergraduate on mechanical and i believe that simple engineering is superior engineering but with these hybrids, they have ICEs and an electric motor along with a Battery. And I was just wondering if i was to buy a 2010 model hybrid, how long would the battery stay efficient before i have to replace it? And how much would it cost to replace one of these batteries?

Thanks.

Oh, and if it makes a difference I don't really drive all that much. Only about 12,000 miles a year. So..200 miles a week give or take.

Suggestion:

What's the first principle of engineering? Keep It Simple, Stupid or KISS. It isn't "A" battery but a collection of batteries on a hybrid and you don't even want to keep it long enough to need replacement. The cost is what they don't talk about. Believe me, it would ruin your whole year (not just a day) if you had to purchase one. Same goes for the plug in electric cars.