When do you know it is time to leave or retire from service?

When you ponder these questions, think about yourself and how it affects you.  We often make comments about the old-timers needing to move on and make some more "room at the top".  This is fine, but have we considered the loss of experience and skill that leaves?  Has that skill been passed on and is the person truly finished with their contribution to the country in this capacity?  Are you considering this?  Completing your commitment to service and moving on after 3, 4, 6 or more years is honorable and very respectable.  Even if you serve and don't like the military, do your job well and leave with honor and dignity.  If you love the service and lifestyle, when is it time to exit?  Opinion or conjecture....