I have loved every bike I have owned and keep them for a while. I really regret the loss of my '91 Harley FXRS. I really like my twincam sport, but can see a day when I am ready for something new. My wife's Evo Superglide is probably a keeper, but if she wanted a new bike, it would go. We have both talked about "someday bikes" and what we would be riding years from now, and we both agree that whatever else we own, the Monster is staying in the garage. Fun, exciting, simple, beautiful, cool and just feels right (hmmm, many of the same qualities as her FXD).
I guess the answer is until one of us can't ride anymore. No offense to the Monster, buy I hope it goes first.