Took a 10 second google search to find your answer. Wonder why you could not have done that?
"The battery pack in any hybrid Toyota is warranted either for 10 years or 150,000 miles (in states with California emissions laws) or 8 years or 100,000 miles in all other states." And, he adds, "There's no pro-rating at all. The battery will be replaced at no cost if necessary during the warranty period."
Toyota says the battery pack is one of the least-frequently replaced items across all Prius models. And some proportion of the packs it has to replace haven't failed, but were damaged in collisions or other accidents.
That was from May 2011.
Found a 2018 Answer from a Toyota dealers website....Toyota Prius Battery Warranty
The battery on Toyota hybrid models, such as the Toyota Prius, is engineered for long-term performance. Thus, drivers around Cerritos will find impressive warranty coverage on their hybrid model. In the state of California, the warranty for a Toyota Prius is good for 10 years or 150,000 miles, whichever comes first. In other states, the hybrid battery warranty is good for 8 years or 100,000 miles. However, it is not at all uncommon for a Toyota Prius battery to last upwards of 200,000 miles. Of course, it depends on your hybrid driving habits.
The start date of warranty work is the date the car was purchased brand new and any Toyota service department can tell you that date with the vin #.
Assuming you are not in CA, the 8 years is almost certainly expired as I cannot imagine any new car dealer sitting on a 2010 model past 8/11. The prior owner may have had the same concerns about the battery as you do. That's why he sold or traded it?