How long can an immigrant with a green card stay out of USA ? Is there any way to stay longer ?

I am 16 years old. I am originally from brazil . My mother is in brazil with my grandpa because he is very sick but also because he Driver's License is suspended so that means she cant work here in america because she is not graduated so the only jobs she can get are jobs where she is required to drive . so we were thinking about going back to brazil until her Driver's License got functional again. we only have the green card because my mother was going to apply for citizen ship in Feb 2010. So can u explain to me if we can stay pout of the country for how long max? Is there any way to make it longer ? where can i find more info about it? And is it a sure thing ?