B2 VISA is the permit to enter USA for vacation. You can travel and stay anywhere in USA if you are approved of the visa application. The validity of the visa is said to be 6 months from the time it has been issued. You will be prior warning as to how long you can stay in that country and when the time is for you to leave USA. You can enter USA for various purposes medical treatment, vacation or a business trip.
If you have entered USA then you have to leave the country on or before the date of your visa expire. If you are not able to then you need to extend your visa. Initially you have to apply for the visa in Form I-94. To extend your stay you need to apply for a visa extension. For this purpose you need to apply for extension by using Form I-539. This will be submitted to USCIS. It is not a compulsion that your application will be accepted immediately. It is completely in the hands of the counsel whether or not to approve your extension. In case of rejection of your visa you have to depart from America. If you cannot depart in time then you will be out-of-status. Therefore it is important for you to be careful of the procedures.