"How long should your retirement last for you to feel it was worth sacrificing for?"
I never thought about it that way. In my early 30's, working for a megacorp, I set a goal to become financially independent and able to retire by age 55. My thinking was I wanted the option of to leave the workforce and live comfortably if I no longer enjoyed working or was forced to do so due to losing a job or health problems. By age 58 I was working for a small company and was unhappy in my job. I decided to retire and turned in my resignation. The owner of the company begged me to stay to help the firm through some legal issues he had caused by making what I thought were poor decisions. I agreed to stay for six months at which time he asked me to continue. I stayed another six months. One day I woke up, decided I was wasting my life, called him and told him I would not longer come into the office. I have not looked back.
Since leaving the working world, six years ago, my wife and I have enjoyed the home we built over a decade ago, our family, and traveling in the RV we bought a year before I retired. The RV experience has been much better than we anticipated because we've made many new friends on the road. Our life experience is truly richer for it. My only regret is the extra year I worked to help someone who didn't appreciate the loyalty and time I gave him and the company. I didn't need the money, didn't enjoy the work, and wasn't happy. I should have been selfish and left on my own timing.
To me retirement is a new stage in life's journey, not a payoff. I enjoyed most of my working years and received monetary, emotional, and intellectual compensation for the years I worked. Those years are behind me now and I'm much more aware of mortality today than I was during my working years. I'm in my mid 60's and hope to have at least 15 healthy and productive years ahead of me. My current goal is to make them as enjoyable and fulfilling as possible for DW and me.
I have no idea how many "good" years I have ahead of me. Perhaps one, perhaps 2-3 decades. I will be happy with however many years I have. I received a full return on my investment of time during my good working years. I can assure you I'm enjoying every minute today.