If you're just dating and you have no kids together, you move on when you discover things about the guy that you don't want in a forever relationship - even neutral things that aren't character flaws, they just aren't your preferences. A guy who loves to party, if you like to stay home. A guy who doesn't want lots of close extended family if you want that.
Whenever it seems not to be working out, take the blinders off and look really hard and realistically at the guy, and leave.
No need to waste your time trying to force a guy or a relationship into something it isn't - find one that IS what you want and hold on to that.
inside you will know when it is time to move on luck jo
If you've exhausted every outlet and tried your hardest to make it work and it just never does. I think after that point, you have to walk away. If feelings aren't the same and you feel like you are just staying out of fear of the unknown or out of guilt because you don't want to hurt the other person. If you are interested in other people and find yourself getting attracted to the point where you don't think you can be faithful. I'm sure there are other reasons, each personal to an individual. You just know!