I need someone with a higher emotional IQ than me to answer this:

Is there an acceptable length to a rant? If a friend calls you and says I just need to vent for ten minutes about something at work, and you say sure, but two hours later she is still venting, and it has become a vent about not just the incident but everything wrong in her life from childhood to now...at what point, if any, should I have stopped her?

I am now even more depressed than usual.