The time is depend on where you want to go in China. 2 or3 days for Hong Kong is enough. Visa is the must, you'd better apply for it in your country, thus can avoid occupying time of your trip. USA Hostel is located in an unsage area, but, as many people said, once you are in the hotel, you are safe, since the safe guard would not let you in unless you show your passport when coming back. The easiest way is to take bus A21 from airport to hotel and vise versa.