How long can you stay outside the United States if you are a Green Card Holder ?

Any prolonged stay outside the US could be construed that your wife is abandoning her permanent residency status. There is no definite number of days but 6 months or more would invite questions. Contact an immigration attorney to help you understand in details the rules and so you can have more definitive answer based on your circumstances.

Your wife should not stay outside the U.S. for longer than 6 months to avoid any issues with CBP at the port of entry.

While there is no definitely timeframe, a stay of 6 months or more outside of the United States could lead to the issue of abandonment of permanent residency status.

6 months is a good max, 1 year is possible but you got show reasons. More than a year, gotta have a reentry permit.