Warranty on replacement phones
The original devices warranty or 90 days, whichever is longer.

A replacement product or part, including a userinstallable part that has been installed in accordance with instructions provided by Apple, assumes the remaining warranty of the original product or ninety (90) days from the date of replacement or repair, whichever provides longer coverage for you.

http://store.apple.com/Catalog/Images/worldwidewarranty.html