I have to go with Zundfolge's response.
My mom was a radio dispatcher with the California Highway Patrol in the '70's. She said that if she ever caught me on a "morta-cycle", she would kill me. She had to work WAY to many cycle accidents at work. She didn't want to be on duty when I bought the farm, as they say in aviation. "The cycle NEVER wins", was her mantra.
My answer is four or five years after the guy that bought it new.
It never ceases to amaze me that you can find a bike that's four or five years old that has under a thousand miles on it.
Seems after about four or five years the guy realizes he really never rides it and decides to sell it.
Ray is right. I once bought a 6 year old bike with 360 miles on it
I always try to buy at the end of the model year or when the new models are released if trying to buy new.
If buying used I like to wait until right before Xmas . Enough time that is been cold and the bike has been parked long enough for the owner to loose interest and close enough to Xmas that they probably need money.
A storm is coming ...