Coming to think of it, it is a topic that no-one really wants to think about. Heck, when you start a relationship or when you are in a relationship, even thinking about when to end relationship is taboo. However if you like it or not, but in your mind you should always have certain criteria failing which there should be an immediate termination of relationship.
1. Cheating - In my dictionary cheating means immediate termination of relationship. You will always come across counselors who will talk about giving second chance and all that. However from experience I know once a cheater always a cheater. A cheater will always find tens of excuses to prove his innocence which exactly makes him a bigger lier.
2. Lack of time - If you notice continuous lack of time from your partner, it is time to start thinking about moving on. There may always be genuine reasons for lack of time, however nothing more is important in life than love. If your partner can find one hour time daily to go to gym, then indeed he can find the time for you.
3. Excessive arguments - Sure every relationship will always have some arguments. However if you two start arguing on thing as minor as what to cook in breakfast and why the other person went to bathroom first, it is time to move on.
4. Money matters - If you think your partner is using you all the time. Your partner never has enough money to pay for his beer, to pay for his car gas, to pay for his lunch, then he has turned into a loser. Who anyway wants to be with a loser?
5. Physical beating - If your partner starts harassing you physically, then you have to seriously think about moving. Your soul is far worth more important than how great your lover is in bed.
6. Lack of respect towards your basic rights - If your partner is abusive towards your language, your religion, your color, your parents, it is high time to get a partner who respects you as a human.
7. Common goals are changed - When you start a relationship, situation may have been different. But in due course of time common goals get changed. You may feel you love living in Houston, but your partner wants to move to Boston. Sure either change the goal or change the relationship.