Fleet manager here, who has had \~1000 Prius go through the fleet over the past 15 years or so -- they're far more age and heat limited than mileage limited. The electrolyte inside the cells eventually dries out and becomes less effective. If a car has been primarily stored in a garage, expect it to last to (or beyond) these upper limits. If they've been stored outside, the lower limits.

1st, 2nd gen Prius, Prius C: 14-20 years, no mileage limit. These cars are so easy on their batteries that they really don't cause them to heat up much. They don't discharge or charge the batteries at high rates of sustained current flow.

3rd gen Prius, Prius V: 12-15 years or \~250,000km. These use the same cells as the 1st and 2nd gen, but are more aggressive with their charge and discharge rates. Curiously, you can take a "failed" battery out of a 3rd gen, put it into a 2nd gen and run it for another 3-4 years.

4th gen / Corolla Hybrid: we've never seen one fail. The oldest cars are only 10 years old and none have shown any issues at all.

There are drop-in Lithium replacements for all generations of Prius which improve fuel efficiency quite significantly, by simply having more available capacity -- they can regen more and longer on a long downhill than the original NiMH battery.

The batteries, in general, are very durable, very inexpensive (we've generally bought used packs out of crashed cars - $1200-ish, or $free if you know where to look), interchangeable between 2002-2022. You can open the metal housing of a 2022, pull the cells out and swap them into the housing of a 2005 and it'll work without modification.

Even if you choose to buy a new OEM battery at a Toyota dealership and have them install it, the fuel savings which have resulted by the existence of that battery far, far outweigh the cost of the replacement of that battery - it's like an order of magnitude of savings.