This is not the right forum to post this question. Go to 'Life after the GC'.
Anyway, you can be out of the US for up to 2years if you apply for a reentry permit ( I 131 ) when still in the US.
Other wise, for a GC holder, you can be out of the US for 6 months without problems.
For more than one year , and without a I 131 , you loose your GC.
For trips of mpore than 6months but below one year ( with no REP ), you are in a gray area : YOU would have to prove yoir ties to the US to the IO.