As a green card holder, how long do one have to stay inside the USA to not lose his green card?

In a way, you're asking the wrong question. Although a 1 year absence will allow the government to assume that you abandoned your residence, they look at other things. Such as whether or not you had 'established' your residence ... this usually impacts people who leave the US shortly after getting the card. They also look at whether or not you are working outside of the US and/or sending children to school outside of the US. Talk to an attorney in private via Skype for better personalized guidance.

As a resident of the U.S. you are obligated to maintain a residence in the U.S. If you remain outside the U.S. longer than six months then CBP can take the position that you abandoned your resident status and you will have the burden to prove that you did not abandon your residence.

Absent any factors which will make you removable, you should not lose your green card as long as you reside in the U.S. An absence of more than 6 months can cause an assumption that you have abandoned your U.S. residency.

I have known several cases when the LPR status was cancelled when a foreign national used the green card as a visa free regime to merely enter the United States twice a year. LPR means Lawful PERMANENT residency.