I've found the very same FAQ's in Lexus.co.uk (quoting):
"- How long will a Lexus hybrid last?
The Hybrid Synergy Drive system is designed to be as durable as any other Lexus, which considering Lexus' reputation is a pretty long time.
- what's the lifetime of the hybrid battery?
The high voltage battery used for the Hybrid Synergy Drive cannot be compared with conventional car batteries. It uses state of the art battery technology and is designed to last for the lifetime of the vehicle. In the unlikely event that your battery needs changing, your Lexus dealer will take care of that."
That's all they have to say about the subject at Lexus.
To me, it means:
1) That there's no way of telling exactly how many years/miles the batteries will go, because that will depend upon too many different variables
2) That, regardless of point 1), they are quite confident that replacing batteries is not going to be an issue any time soon, not even for the first wave of second owners, among whom I'm eager to be.
I don't think the lifetime of the batteries should be a major concern if you're considering to buy this car: much better question is, given my usual mileage, traffic condition and average speed, can I make even with the higher price of this model in X years?
I'm afraid the answer will be NO, for the 99% of us.
I do love the car, but I believe the only rational reason to buy it might be to skip traffic blockades or congestion charges (like the one you have in London) on environmental basis.
Apart from that, it's just a wonderful, wonderful toy.