I was a greencard holder.. If i recall correctly, my lawyer told me I need a I-131 if i plan to leave the country for an extended period of time (i.e. over 6 months).
I believe if you keep EACH STAY less than 6 month, u don't need it.

i left for 1.5 years so I did apply and obtain one. However, if you obtain one, your counter for citizenship resets I believe (so you'd have to do 3 more years after you return from your extended stay).

This is what i recall, and certainly I'd consult an immigration lawyer to be sure.