I am U.S. Green Card holder. If I travel out of USA to my parent country, How long I can be out of USA and within how I should enter to USA.

As far as I know, you can stay up to 1 year outside US. For periods between 1 year and 2 years, you need a re-entry permit. However, as far as I know, any absence longer than 6 months breaks continuity -- you need 5 years of continuity where you spent at least 6 months/year in the US and you have not spent at any time more than 6 months outside US.
:)
However, together with your GC that you don't really want but somehow wish to keep, you might have received a "Guide to Naturalization". If not, you may want to read it here: http://www.ailc.com/services/natz/English.pdf