Away from the U.S. less than 180 days, no problem. Between 180 days and a year, maybe a problem. More than a year, definitely a problem.
You should plan to return home (the U.S. is your home, right?) before the 1 year mark, and be prepared to explain why you were away so long and show evidence that you have not abandoned your residency.
It's important to note that CBP might try to pressure you into signing an I-407, which essentially is you admitting that you've abandoned your residency. You do not have to sign this - as an LPR you have the right to be admitted or paroled into the country pending a court hearing where you and your lawyer can show that you did not intend to abandon your U.S. residence.
Then again, it's reasonably likely that you'll be admitted without any hassle (it all depends on the CBP inspector and what she had for breakfast that morning) so try not to worry too much.
References: The legislation, regulations, policies and case law in this area are rather complicated and easy to misinterpret, and IANAAILAL. However, the 180 day threshold comes from INA § 101(a)(13)(C)(ii), after which an LPR alien is "regarded as seeking admission". The 1 year threshold comes from 8 CFR § 211.1(a)(2), after which an SB-1 returning resident visa is needed, which is not easy to get. Further discussions: 1 2 3