When Is Enough Enough?

I am sure the title of this blog is gramatically incorrect but I am beginning to believe there is nothing correct about relationships. I believe you need to set personal boundaries and draw the line when they are crossed......repeatedly. A variety of excuses is where a line needs to be drawn. An excuse is given and yet the other person ends up doing something completely different and not even remotely close to what they told me they were going to do. What is the difference between an excuse and a flat out lie? Evidently it is a gray area to some when it should be more black and white. When do you throw in the towel and say "enough already?" I am not getting the emotional investment out of this that I am putting in.