If you plan to stay more than a year but less than 2 years you must apply for a Re-entry permit before leaving the US. IF you stay longer than 2 years you can loose your green card since you are breaking the "residency" term since is suppose that you apply for residency is to live and work in the US not outside, unless you are in an official US goverment/armed forces type of job.
Check the part at the uscis.gov when it says "now that you are a permanent resident".
Good luck,