The minute they prove to be unworthy (I.e lying) or your common ground disperses. I was always taught 'you're only as good as the company you keep' so I associate myself with like minded and driven people who share the same interests as I do - I just can't be around people who have no direction in life or share none of my interests.
To give you am example of someone I cutoff recently, we had been best friends for the past 7yrs, inseparable if you may. He was a tradesman and I was the academic type. He changed jobs and all of a sudden hit the drink and gambled ALL his hefty weekly pay checks! At the same time this happened he would skip on our catchups to go to the local bar and would be uncontactable for days at a time - his own Gf and family would call me to ask if I'd seen him! I'd done a lot for this guy and looked at him like a brother but the final straw came when he skipped my birthday and has the balls to lie about it when I caught him out - told me he came to the bar I was at at about 12am but couldn't find me.... I was the until 3am and didn't see him once, which is funny cause the place was pretty small. Anyway, a friend of mine told me he saw him at another bar at 1am still drinking - I would've been mad but forgiven him if he had just told the truth but enough was enough. We no longer talk and I'm better off