Okay after 8 years my main battery will die?

1) In general, Manufacturers plan on 5% of all products failing within the warranty period.

2) At one point in time, Toyota mentioned that 1 in 40,000 HV Batteries had failed within the 100,000 mile warranty period. It is not clear that this is still so.

3) I hold the contrarin view that the HV Battery is more pampered when on than off, so the more you use your Prius, the longer the HV battery will last. In taxis, one hears of 300,000 mile life, but taxis run 24/7.

4) The retail price of the HV battery has always trended down, so there is no good reason to buy one before you need one.

5) Don't buy retail, rebuilt batteries are available for $1500, and used batteries are available for $300 and up. So long as drivers are so bad that more drivers wreck than wear out their battery, there will be used batteries.
Prius Battery items - Get great deals on eBay Motors, Charging Starting Systems items on eBay.com! (As I look at this today, I see $400 as the lowest current price, but Toyota offers a $200 recycling price, so it never goes below that)

6) Search is a wonderful feature, you might have avoided being the thousandth person to ask this.

 

I'm pretty sure that the main factors in battery life will be cycles (number and depth of) and heat.

One thing I've noticed about the Prius is that cold starts can put a fairly big discharge/recharge cycle on the traction battery, especially if you drive straight away without warm up. Also most taxis will be wont be working in mountainous terrain so it makes sense that they can put on lots of miles without cycling the battery too much.