B-2 visa for chinese is normally valid one year for multiple entry. the legal principle is every visitor is assumed to be a immigrant. visitor has to prove that she has no immigration intent . she already convince the consular officer that she do not have immigration intent and get visa but legally immigration office in airport has authority to assess immigration intent independently again. if you face normal officer, she should be asked for her purpose of trip and length of stay, and she will get 6 month stay. but across us border is crap shoot, she might face some a$$, her immigration intent might be assessed again. Thus she should take her all document for his visa application and a return ticket in case of problem.