When to replace timing belt?

I know I can call the dealer on this (and most likely will), but what's the frequency of those of you with higher mileage Lexuses normally change your timing belt?

I see many people with well over 100k on their cars that post to this group and I'm just curious if or when your changed your belts?

I was looking at some earlier model SC300's the other day and most of them are routinely in the 130-150k mileage range and, unless they have a Lexus maintenance history, it's highly likely that if I had purchased one, I'd have to expect to pay to have the belt change. And, for those of you that *did* get a belt change, did you do it at your dealer or have an independent shop do it and how much did it cost?

TIA!