For as long as they can. Narcissists need people. They have to have someone to envy them; agree with them; think they are they greatest thing in the world and so on. You are pretty much in their world to be what they need you to be not what you are. They only act of kindness you get is when they notice you have had about enough of them and since they don't want someone to leave their life they throw you something to make you feel special for a bit; then its back to them. And if you do decided to leave well its YOUR loss of course.