Technically britsimon is right that it is a year, but there is something about a 180 day "rule" - if you re-enter within 6 months you normally will have no problems,but longer than that and you are likely to get questioned quite closely by CBP to determine whether or not you have abandoned residency. They usually understand though that people need time to sort things out before a permanent move. The CBP officer cannot refuse you entry or take away your green card, so don't worry about that, however if he feels you have abandoned residency he may institute proceedings to make you argue your case before an immigration judge. However this is highly unlikely to happen on a first reentry as you will make, but they may annotate "advised residency requirements" in your passport which means that if you keep leaving for longish periods you will find more trouble. Try to establish some ties in the US on your first landing such as bank account and keep an address there. Most important : you landed in December, which means you are required to file with the IRS for tax year 2013. Make sure you do this, as this is one of the key tests they use for whether or not you have abandoned residency. (And look on the irs website for what to do when it is your first year filing if you have earned income elsewhere)
Short version: you are ok with a year but six months is much safer and make sure you can show you always intended coming back.
Also,be aware that if you stay out longer than 6 months the time counted for naturalization resets to the later date, whereas if you come back within 6 months it is still counted from your first entry date.
