Can us green card holder stay over 6 month overseas?

After a year in US as a green card holder in case for example you need to stay outside US for employment reasons then you can file Form N-470 to preserver residence. Trips outside US as a green card holder should not exceed 180 days per trip. If you are going to stay outside US for more than a year remember to apply for reentry permit valid for 2 years and get it approved before leaving US.