I don't think that there is really a hard number for this. There are some people who buy a sportbike, ride the #$%& out of it right away and junk the thing. Others may break it in right, put a lot of miles on it but it will be in much better shape than the first guy's. Knowing the bike's history is probably the best info you can have on a used bike. Dunno if this helps much, but maybe some? 
------------------
"We don't got any beers. We got tequila, it's like beer."
------------------
"We don't got any beers. We got tequila, it's like beer."