Generally the maximum without a reentry permit is one year.
However, staying outside the US for less than a year could result in losing your green card if you also took multiple long trips in recent years, with short stays in the US in between (for example 6 trips of 5 months each with 2 weeks in the US between trips). Or if you worked abroad in an open-ended arrangement (i.e. not a short-term contract with a fixed end date). Or if you acquired permanent residence in another country.
And there are circumstances that allows staying outside the US for more than a year without a reentry permit -- working abroad for the US government or military, or accompanying a spouse who works for the same.