hi,
my dad got a visa for 5 years. Is it true that he has to go back after 6 months? Can he stay for 5 years?

plz suggest me.

As far as I know, if your dad received the visitor visa, then he can only stay in US for less than 6 months. It has to be LESS THAN 6 months, can't be a day more than 6 months. However, he can travel back and forth from Nepal to USA at anytime during those 5 years of the VISA (as long as he continues to stay in US for less than 6 months).

yes he can stay only less than 6 months and he has to get visa every six months , does ppl go to canada or mexico for reentryinstead of coming to nepal?

Microbilology, as far as i know, you can request to extend visitor visa to 1 year. but , you have to have valid reason.   

http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=94d12c1a6855d010VgnVCM10000048f3d6a1RCRD&vgnextchannel=db029c7755cb9010VgnVCM10000045f3d6a1RCRD   

   http://www.path2usa.com/usa-visitor-visa-extension

hope that helps.                                  

It is not always 6 months. It depends on how much time they stamp on your passport at port of entry. Some are given 90 days some 6 months. You can always file for an extension of stay before the expiration date. The 5 year visa is not for 5 years stay but the ability to enter the US for 5 years. The amount of time you can stay in the US is determined at port of entry each time you enter the US.