That's kind of like asking when does a business start making money. For some, it could be next month, for others it could be next year. It depends on the traffic, how you intend to make the money, your visitors, your competition, many different things.
As to "how many users, page views, etc. does a site need in order to make money", that's a question only you can answer. How much are your expenses? Once you meet all your expenses you will then begin to "make" money. For you that may mean 50 page views, for MySpace it may mean 50,000 page views.