How frequently must batteries be replaced?

I think the Prius comes with either an 8 or 10 year warrantee on the batteries so might be worth checking with your local dealer.

The Hybrid Battery is designed to last the life of the car. And as far as I know, a Toyota Hybrid System battery has never had to be replaced due to wear...replacements have only been done if in accident or defective from the factory to begin with. There's a separate smaller battery used for starting the hybrid system that should be replaced about as often as batteries in regular cars. Don't worry, these are very low maintenance cars...you'll only be replacing/rotating tires and changing oil. If you drive it for mpgs, you may not even be changing brakes. My step dad had 230k on his 2005 Gen II before he needed to replace brakes. I had almost 100k on my 2008 before trading it in and the brakes were a little less than half gone. If you are asking because you are considering purchasing one, you won't regret...they are amazing cars!

Matt's answer is correct. The regular battery may be 4 to 7, but mine is still fine at 5 1/2 years.