Permanent Residents can stay out of the US for a maximum period of 2 years with reentry permit. Bear in mind that the permit must be obtained in advance before traveling. Also, one will not be able to keep applying for the permit every 2 years unless you can show USCIS some unexpected and unusual circumstances.

Like someone said earlier, this will reset your date towards obtaining citizenship; if you intend to apply in future.

Also, if the USCIS sees the trend of merely maintaining permanent residency but not actually residing in the mainland US, they can revoke your permanent residency status.