Friends are not always true friends. True friends are friends who believe on you, who treasure you and appreciate the best on you no matter what. They are there in your ups and downs. In short, they cheer you up for you to smile!
When to leave a friend?
It is when they don't find a value of your presence and something that you do.
It is when you describe them as user-friendly friend. It means they are just there turning as good friend because they need something from you.
It is when they start rejecting you and taking you as worthless to spend time with. In short, if they don't initiate communication to you. Don't carry the burden, just simply let them go!
It is when they discourage you instead of encouraging you; when they demotivate you instead of motivating you; and when they don't appreciate your achievements instead of appreciating you.
Remember: God loves you no matter what. He is your best and true friend.
P.S
This is just my guide based on my life's experience. I observe and even share it for you, even if you find this unworthy and I admit it.