For the HV battery, miles is not the measurement it is charge cycle ... The charge cycle is not closely coupled to miles driven !!!! Long highway drive with a lot of miles, may tax the battery a lot less than stop and go city driving with a lot LESS miles. But even taxis, which are one of the worth possible use case, last longer than 100k (usually).
We purchased our 2010 Prius in May, 2009. We have 115,000 miles on it. I have noticed no battery degradation at this time. Here in Oregon the battery is warrantied for 150,000 miles. If the battery does become unusable I hope it occurs before 150,000 miles....
dealer told me you need replacement after 150k miles, but i dont know if its true. they say anything to sell you a car