how responsible are we to our friends?

You were completely reasonable to draw the line when it comes to work. If someone is calling you all day during work, there's an unhealthy relationship there. It sounds sort of like dependency. You could tell her to send you emails so that you can give your attention to her when you have the time, or tell her to find more friends so she can vent to them sometimes, instead of taking up all your time.

I think friendships should be held on equal footing. A give-and-take situation, if you will. That even though one can use the other as a means to vent, it should be realized that this is an imposition. That there are certain liberties that the ventee is taking away from the person listening to the rant. And as much as your friend wants you to see it her way, she should also see it your way. Or what she is putting you through by listening to her.

I had a friend like this also and it was so out of hand. I use to spend 1 - 2 hours completely listening and trying to advice. The thing is that the more I listened and advised the more she would call about the same issue. Don't get me wrong, I love her dearly but it was so out of hand that she would call me at work and while I was in meetings and she had to leave messages she would call the main number to get me interrupted from meetings. I spoke to her numerously that she had to cut me a break while I was at work. She never understood and would be upset thinking I was not a good friend. So I had to sort of train her a bit and ignore some of the calls. Funny thing is that she will stop for a while and restart up again. Although I am not working now, but she is I can't help but wonder how much work she can't accomplish because she is always on the phone. So I feel I have gone beyond my responsibly as a friend but some do not understand at times.