how many miles on '99 zx-6r?

Depends on how it was ridin before and how you plan to ride it? I have 15K miles already on my 01 brand new and she still runs great, on riding scale of 7/10 10 riding it like the star boyz and thrashing the bike. I wouldn't expect to start losing power until about 30/40K who knows how long a sportbike would last, most people trade often or just plain total it in a few years.