One of the requirements for maintaining GC is that one has to be six months in US or rather one cannot be outside US for more than 6 months.
But those six months is counted from January or from the date of entry into US?
There is no such requirement, but there are several ways to (perhaps inadvertently) demonstrate you have abandoned your US residence, and thus forfeit your LPR.
You may void your LPR under the following circumstances:
1a. Remaining outside the US for a period in excess of 1yr, without the benefit of an approved I-131 Reentry Permit.
1b. Remaining outside the US beyond the 2yr protection given by a Reentry Permit.
2a. Moving abroad without maintaining close ties to the US. No specific timeframe required, as this one is based upon demonstrated "intent" to abandon residency.
2b. Moving abroad and taking up foreign employment. Again, no specific timeframe required as it's an "intent" based problem, but generally this'll be harder to rebut than the case where someone is simply living abroad.
3. Being convicted of a removable crime, especially a CIMT (moral turpitude).