FOREVER!! its hard enough to get permission from my wife to get anything new the way it is so when I do get permission to spend money it better last forever. Her opinion of bikes.

My opinion you can never have enough toys. I believe there is no reason to replace a bike unless it not safe or not rideable any longer. If you get a quality frame you will never be without a ride unless there is a frame failure.

BTW I am currently building bike #4 lol

It really comes down to budget, desire, and personal choice. Do what you want but there is no reason to replace every X amount of years just because.