How do we ever know when the time has come to end something, stop it or move on? Careers, relationships, hobbies (knowing when to stop editing a book)...these are things people nurture, tend to, and watch change as we move along in life.
I've seen many examples of people that knew exactly when their time was up. Certain singers who have called it quits because they realize they can never be as good as they were in their hey-day. Politicians who have quietly moved back into regular civilian life. Teachers who have reached the end of a very fulfilling career having seen numerous students through the door, but knowing time had come to step away from the text book.
But for all of those, there are others who just don't know when enough is enough. Certain celebrities who have a 'farewell tour' every other year. People who are knocking on a certain age, but insist on doing things they did when they were younger instead of giving someone else a chance to take on that role. And writers who will continue writing even if publishers refuse to take on their literary works at their publishing house.
But then again: it's those that don't give up that we end up hearing great things about. I suppose if you've found something you do well and you love - maybe you should continue doing it until your own body will no longer allow you to do it.
The barometer that says "it's time to stop".
I plan to keep moving until that internal clock tells me time is up.