I assume you already are a GC holder :
Please post your question in the forum "Life after GC and US Citizenship ", you would get better help there. Here you are in the forum for people dealing with obtaining their GCs through the Lottery.
Anyway, here's a snapshot of info:
_ trips < 6months: no problems with GC, continuous residency ( for naturalization ) not broken
_ 6m < trips < 1 year: gray area ( continuopus residency is likely to have been broken, burden is on
YOU to show your ties to the US so that you did not break the permanent resident intent )
_ trips > 1year: GC is broken ( unless a reentry permit was obtained ), and continuous residency is broken ( unless form N470 was obtained )
However I would use caution with those numbers: the immigration officers can question your intents for
ANY period. ( and there is no true difference between being out for 181days instead of 183days, it is about 6months, which is a long absence for a LPR )
To show your ties to the US, there are a few things: ( they may not all apply to your case )
_ file US taxes ? ( and a resident, not the forms NR ) ( if yes, get a tax transcript, it's free )
_ where was the rest of your family living ?
_ were you still having a residence in the US ( owning or renting a place ) ?
( or were you using a friend's address ? )
_ belongings in the US : cars, furniture...
_ activity in the US : bank accounts, credit cards, 401(k), IRA...
_ pay stubs ( US payroll )
...
Be also prepared to prove that you intended to be out temporarily ( Getting a degree is a "good" reason.)
( why did you have to stay longer ? ) ( if you still have those, show your intial return plane tickets )
http://www.hooyou.com/reentrypermit/...glegalPRS.htm
http://www.immihelp.com/greencard/reentry-permit.html
http://www.uscis.gov/files/article/B5.pdf