see
http://www.toyota.com/vehicles/2005/prius/faq.html, Question 17:
"Hybrid-Related Component Coverage: Prius' hybrid-related components, including the HV battery, battery control module, hybrid control module and inverter with converter, are covered for 8 years/100,000 miles. The HV battery may have longer coverage under emissions warranty. Refer to applicable Owner's Warranty Information booklet for details."
As taken from the 2004 Toyota Prius Owner's Warranty Information booklet (PN 00505-T2004-PRIUS) page 6, "Warranty Coverages at a Glance" there is a bar graph identifying the various warranties (from my 7/03 printing):
New Vehicle Limited Warranty:
"Basic: 36 months / 36,000 miles
Hybrid Vehicle System: 96 months / 100,000 miles
Powertrain: 60 months / 60,000 miles
Corrosion Perforation: 60 months / unlimited miles
Restraint Systems: 60 months / 60,000 miles
Emission Control Warranty:
Federal, Performance: 24 months / 24,000 miles
Federal, Performance Specific Components: 96 months / 80,000 miles
Federal, Defect: 36 months / 36,000 miles
Federal, Defect Specific Components: 96 months / 80,000 miles
California (t), Performance: 180 months / 150,000 miles
California (t), Defect: 180 months, 150,000 miles
California (t), Hybrid Battery Pack: 120 months / 150,000 miles
(t) = Also applies to Maine, Massachusetts, New York and Vermont vehicles equipped with a California Certified Emission Control System. Vehicles covered by this warranty are also covered by the Federal Emission Control Warranty."
So, federally, it's 8 years/100,000 miles on the hybrid battery in the US, unless you have a 2004+ Prius and live in a CA emissions state, where it's 10 years/150,000 miles under the emissions warranty.