You can stay out of the US for up to 6 months with no consequence, as long as the person has a vaild green card and passport. Over 6 months contact with the US consulate is needed and the green card holder can stay out of the country for up to a year. Over a year the green card holder will forfeit their green card and not be allowed to re-enter the country.
- The US permanent resident CAN stay out of the country for more than a year if they apply for and are granted a re-entry permit by USCIS prior to leaving the country. However, for a PR to become a citizen, he/she must have 5 years of continuous residency and staying abroad for more than a year will typically break it regardless of whether a re-entry permit is issued/used.