Basically, never. In fact, no one knows the MTBF (mean time between failures) of the traction battery because there aren't enough Prius out there with 300,000 to 400,000 miles on them.
If you should ever have to replace it (and the warranty is 150,000 miles in CA emission states--100,000 miles elsewhere--it's not a pro-rated warranty either) the current cost for the 2008 Prius is $1,967.39 plus about $600 installation. However, this price has been going down every year. In addition there are companies that can refurbish the battery for an even lower price.
The Prius' battery just isn't something you need to worry about.
FWIW, my 2001 and 2004 both have the original batteries.
UPDATE:
For those who say the Prius is expensive to maintain. Well, they've obviously never owned one. Now I'm sure that out of the over one million Prius sold there will be a few that cost more than they should to repair. It's a car not a panacea. However, the vast majority of Prius owners, including me, just drive it and change the oil occasionally. Dealer service at Toyota is not particularly cheap but it's no more expensive for the Prius than it is for other Toyotas, and compared to the VW I had before the Prius it's about 1/3 the cost.
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