Rights and obligations for green card holders?

First off - "green card" is not a legal term. The correct term is "legal permanent residency" and describes an unlimited work and residence permit, with the focus on immigration.
By issuing an immigration visa, the US does not invite a foreign citizen to flexibly spend time in the US or work there temporarily for some time. An immigrant is expected to live in the US permanently, in return he or she is granted a considerable number of rights. Only then does this status make sense in comparison to the non-immigration visas.

In principle, the green card has lifetime validity. However, each US visa does not only entail rights, but also obligations for the visa holder. Therefore, it is also possible to lose a green card again.

For example, if the border official has reason to suspect that the green card holder has given up his or her permanent residence in the U.S., this may have some rather unpleasant consequences - including revocation of the green card.
Usually, this will be suspected in case the green card holder spends too much time outside the U.S.:

spending up to six months outside the US should not present a problem in this context
even if you spend between 6-12 months (less than one year) outside the U.S., your green card will still be valid. However, an absence that long may already lead to some unpleasant questions from the border official.
After you have spent an entire year, i.e. a continuous 365 days, outside the U.S., your Permanent Resident Card will no longer be valid for admission into the U.S.!

However, there are two ways you may keep your green card even if you spend more time outside the U.S.:
Re-entry permit (travel document) or Returning Resident Visa.