When the relationship is pretty much over, and there are times you don't even like each other -- but you still love him/her, it's probably time to call it quits.
If you're basically just friends, (rarely intimate) and you've just become a habit, and at times dread even seeing the other person, it's time to say goodbye.
If you find yourself attracted to others, but you don't act on it because you don't want to cheat -- but yet you really don't feel you're in a 'relationship' then it's time to hit the road.
It's best to break up while you're still friendly than wait until you have a horrible fight, that ends the friendship forever.

Best of luck and God bless. Hugs, Lee