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.