When is enough, enough?

Not just a post but a question to all those who follow me. When is enough, enough? When do we stop giving to those who only take and never return in kind or in any kind for that matter the gifts you offer them. I am not talking celebratory gifts like birthdays and weddings where once in a while you go way over board for whatever reason and give extremely generously. (by the way people expecting re-gift visa vi your exuberant spending is wrong on all fronts). I am talking things like patience, going out of your way to do a favor you were asked to preform and against your better judgement or based on your friendship to that person, you did it as a "favor". When does one have the right or balls if you will to say no because you the person in question feel you have done enough for this individual? Is there a guideline to follow, is there an agenda somewhere that lists the check list of "do's and don'ts".
As I get older I find my patience for stupidity, arrogance and expectations of what should be right for all to be lacking at best. I find myself angered often by stupid ill thought out comments by juvenile minds in the bodies of adults or ridiculous self center actions. At times I grieve for my younger years of frivolity actions and precarious activities where though i was to be held responsible for those actions they were nonetheless my actions and it was an exciting time. I guess I know how my mentors felt when they observed me and asked the same question "When is enough, enough?