This is one of the most difficult questions in the field of immigration law. In reality, there is no time limit, although there are some milestones where things start to happen. After six months, you will be presumed to have broken physical presence for naturalization. You will also have no presumption of admissibility back into the United States. After a year, the USCBP will expect you to prove to them that you did not intend to abandon your residency. I once saw a case where a resident had been out of the United States for 7 years, but was able to retain his residency, but that was a highly unusual case with some compelling humanitarian factors.
So, some things you should do. Plan on filing an I-131, requesting a re-entry permit. This serves of a declaration that you intend to return. You need to be in the United States when you file this. Second, make sure you have proof of your intent to return. Assets and property in the United States go a long way towards this. Third, consult with an experienced immigration attorney about your specific situation, and get concrete answers about other actions you can take.
This is one of those times when spending a little money up front can avoid big headache later. You don't want to be in secondary inspection at the airport, trying to explain to a cynical CBP Officer why you should be let back into the country. A little preparation now can avoid big headaches later.
Enjoy the sabbatical.
My response to your question is general in nature, as I don't know all of the facts of your case. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. In addition, my answer does not create an attorney/client relationship.
Correct. You cannot stay outside of the USA for a period of time longer than a year, without inviting a host of problems upon yourself including losing your LPR status completely. If you need to stay longer than a year but less than two years, get a reentry permit. Make should you disclose on the application that you would be staying for longer than a year and why.
The information contained in this answer is provided for informational purposes only, and should not be construed as legal advice on any subject matter,not should it be viewed as establishing an attorney client relationship of any kind.