.... I'm sure this is a loaded question, but at what point does mileage become a concern?
How long is a piece of string?
A daily rider will probably clock up 10,000 miles a year. A seasonal, weekend rider might only
do a small fraction of that.
There are many variables to consider. The age of the bike. Type of bike. As oldenslow
says above, sports bikes might have endured a harder life than long distance tourers.
Service history. Oil changes, valve clearance etc....
finding a bike with, say, 5,000 miles wouldn't be a problem.
5,000 miles is very low mileage by any measure. For most modern bikes,
the recommended oil change interval is longer than this.
I've seen a lot of bikes with 10k to 20k miles.
I would expect to see this kind of mileage on any bike 1-3 years or older.
If the mileage was much lower than this on an older machine, I would suspect
that the mileage had been tampered with "clocked".
....but also a number with considerably more.
That shouldn't be a problem in most cases. We take it for granted that
cars or trucks will run for hundreds of thousands of miles without any
major issues. There is no reason why bikes can't do the same.
My old Kawasaki has only done 76,000 miles. I would be very disappointed
if it didn't last for twice that distance. My friend's Honda Gold Wing is at
140,000+ and he is planning to ride it to Russia this year.
There are differences between types, models, brands and engine sizes.
A large capacity tourer or sports-tourer should be good for very long
distances. I would have a bit less faith in a small capacity two-stroke,
although there are a few cases of 50-125cc two-stroke engines covering
Maintenance is obviously key,
Oil changes and valve clearance checks/adjustment are particularly important.
Other forms of neglect can often be corrected after you buy the bike.
and buying from an original owner who has all maintenance records would be ideal
In this part of the world, a bike without full service history will sell for a
significantly lower price.