I used to wonder about this when I was a child. One day all on my own I realized this,was silly. Just keep reminding her that the world isn't going to end.
She might just be curious. She might not have any predisposed idea that the world will be ending in some horrible way or anything. Think of it as she sees it. The world is something that has been going "on and on and on" like she stated and wonders when it will end. Everything else in her life ends or stops at one point, visiting, bath time, eating, playing movies, etc etc.
It could be she has over heard some talk by adults , or on TV ..It sounds like she is a bright child , I think its best to reasure her all is well, that you are with her and wont let any harm come to her .Maybe then let it drop, say no more because if its fed into it may keep going .,make sure no family or anyone are scaring her . good luck