How long can my wife stay out of USA with a green card visa?

Up to 6 months is no problem at all. Between 6 month and 1 year you will get extra questions when you return but will probably be admitted. Over 1 year and you will loose your green card unless you got a reentry permit before you left the U.S.

You can't get the reentry permit from outside the U.S. The reentry permit costs $385. If you stay outside the U.S. for more than a year it will restart the clock for naturalization.

If you want to read more details about this, check out the link below.

Glossary of visa terms:

Re-entry Permit: A travel document that the DHS issues to lawful permanent residents (LPR's) who want to stay outside of the U.S. for more than one year and less than two years. LPR's who cannot get a passport from their country of nationality can also apply for a re-entry permit. You can put visas for foreign countries in a re-entry permit.

http://travel.state.gov/visa/frvi/glossary/glossar... ..

http://www.uscis.gov/portal/site/uscis/menuitem.eb... ..

The card is valid for return after an absence not exceeding one year.