First of all, as far as Generosity goes and your energy going down when you are helping someone else, it would be good for you to investigate to see whether you are wanting a result rather than just giving for the sake of giving. Some things are beyond our power to control and if we are helping, being generous, and we are wanting that person to show the result we want, then we can get very drained. However, if you have equanimity protecting your generosity when helping people, then there is not so much of an aspect of getting drained. In this case helping people is good for you, too. This type of Generosity is good for both people.
When should you say no or stop? That depends on the other person and it depends on the situation. If that person is, say for example, an old student who comes into an interview and they tell me a Dukkha story and I see it is because they don't accept impermanence, then I say, "Well, this suffering is because you are not accepting impermanence, you need to reflect on impermanence more." Now say they keep coming back with the same story, and I keep saying, "Well, you need to reflect more on impermanence." And they say, "I know, I don't like impermanence. I don't want to reflect on impermanence." Well, sometimes, depending on how many times it happens, I say, "Well, if you want your Dukkha, you can keep it." I don't usually do this, I have much more patience than most people.
However, if you get very excited, "Oh, I know the antidote to this and I give it", and you expect that the person will come back and be cured, you leave yourself open to more Dukkha. People change slowly. And no matter how perfect the seed is that you give, it depends on the soil that you plant it whether it will grow. So my short form of 'to care and not to care' is, 'some things are beyond my power to control'. And I find myself reflecting on that a lot. Especially when I ask people, "Well, have you got a formal practice going yet?" And they say, "No". Some things are beyond my power to control, I can't do it for others.
So when should you say no or stop? It is very difficult for me to say this unless I know the situation, but certainly we can increase the power of our generosity by increasing our wisdom. Generosity needs to be balanced with wisdom, otherwise we can be giving and giving and it may not be good for the person we are giving to either, because they give up their responsibility, they get dependent on us, so we have to give in a way that will help them be more responsible for themselves.