Port Of Entry Procedures for USA Visitors
Applicants should be aware that a visa does not guarantee entry into the United States. Legally, the visa specifies the period during which the bearer may apply at a port of entry to enter the United States. The Customs and Border Protection (CBP) officer at the port of entry authorizes the traveler's admission to the United States and determines the period of authorized stay, not the consular officer.

A person who receives a 10 year multiple entry visa has a 10 year period during which he or she may apply for admission to the U.S. It does not mean one can go to the U.S. and live there for 10 years. At the port of entry, an USCIS official may deny admission (which is equivalent of canceling the visa) to the holder of a valid visa if the traveler's activities will not be compatible with the specified visa category, or the USCIS officer discovers some fraud with the traveler. Legally there are various grounds of exclusion such as certain diseases, previously being out-of-status, criminal history based on which the USCIS officer can deny the entry into U.S. despite the fact that the person has a valid visa. If a person contradicts a statement made at the Consulate, or if there is a discrepancy between the documents being submitted at the Port of Entry and the statements by the applicant, the immigration officer at a port of entry may return that person to his/her home country.

As it depends upon the immigration officer at the port of entry how much duration to give for stay in the U.S.

More details about Admission into United States

Uncomfortable in English
Many people are worried about their relatives who are visiting the USA and don't know English or are not fluent. They wonder how the visitor would be able to communicate with the immigration officer at the port of entry. Most likely, there would be several others in the same plane who would know the local language of your relatives and also English. The immigration officer would usually ask someone from the line to come forward to translate for your relatives. However, it may not always happen and in that case, they would have to just manage somehow. You are not allowed to go inside to help them.

In this case, it is advisable that you write a letter addressed to the immigration officer, explaining why they would require a 6 month stay in the U.S. A sample itinerary for their travel plans would be useful too.

Samples:
Letter for port of entry for longer stay
Itinerary for longer stay

Of course, those are comfortable in English should convey to the immigration officer at port of entry to give them a longer stay, using the reasons given in the sample letter above.

Authorized Stay and Extension
Please stay in the U.S. only till the date given on the I-94 form. If you intend to stay longer than that, please apply for an extension of visitor visa, well in advance, but not immediately after coming to the USA. If you overstay even for one day without extending your visa, it will automatically void the multiple entry visitors visa stamp in your passport. If you plan to visit the United States again in the future, you will have to apply for a new visa in your home country. It may also result in a 3 year or 10 year bar from entering the U.S., and if still in the U.S., the person may also be removed from the U.S.

Also read,
Grounds of inadmissibility
Time bars entering United States