To maintain Permanent residency, one cannot stay outside the United States for longer than a year. If one plans to spend more than a year outside the states, s/he needs to apply for Re-entry Permit.  If s/he plans to apply for Citizenship down the line, she cannot spend more than 6 months as spending more than 6 months will reset the waiting time for eligibility of naturalization application. Hope that helps. 

riddle · Jun 10, 2012 7:33 PM

Thanks for your reply riddle, they don't have any plans to apply for citizenship in future. All they want is to maintain the residence status so they can visit us whenever they want without any hassle. Down the road may be after 10 years when they retire they might come whet and stay for few years but theres no plan of applying for US citizenship

looser · Jun 10, 2012 10:44 PM