Michelin Pilot Sport PS2 265/40ZR18 - How long are they supposed to last?

[wt888usa]

I had about 19,000 miles on these tires (Michelin Pilot Sport PS2 265/40ZR18), which came with my Boxster when I purchased it. I was driving and all of a sudden the rear tire on the driver side went flat, no indication of any road debris damage or any nails etc. Roadside assistance took me to the Porsche dealership where I had to replace BOTH rear tires (something to do with the balance of treadwear) for $960 (!). This was back in April, and it never occurred to me that maybe I could have gotten this covered under the tire warranty? The warranty seems to say that it is covered up to 1.6mm left of treadwear (which seems really low, and I know the tires had more tread than that), how many miles are you guys getting on these tires? Is it too late for me to claim money or tire replacement now that I had done this 2-3 months ago?

Also, since I am now at around 21,000 miles on the front tires, should I be replacing my those as well? I came from driving a Mustang where I almost never had to change tires, so I'm not sure if now that I'm in the 18" performance tire world, I am supposed to be expecting tire changes every 15,000-20,000 miles, even if the tread isn't even that low. Even with the infamous penny trick, the tread went up to a point between Lincoln's eyes and his hairline (:-P).