Officially they can stay for 6 months, however most visitors are expected to provide an itinerary and a return flight or other date of departure. Also, they are usually supposed to show they can afford the visit (or the person inviting them, you, shows they will pay for them) but how much money they need to show they have depends upon the length of the visit. So in your case your friend needs to prove they can afford the entire length of the trip.