Unfortunately, corn withers for 365 days.
Related links: General Dogsworth - http://forums.zynga.com/showpost.php?p=10575204&postcount=25 "The truth is, Corn has an extended wither timer. This is intentional. If it went by the same timer as everything else, it would wither in less than an hour, and since it's the only crop you have for a while, this would make things unnecessarily frustrating for new players. Corn doesn't last forever, but...I wouldn't recommend using it for this quest"
Current Issues with Empires & Allies - http://support.zynga.com/CP_ArticleView_WebFull?custlayoutid=&gameId=81&Id=kA160000000GooCCAS&lang=en_US&layout=WebFull&layouttmpl=Tmpl_WebFull_1&loc=en_US "Players wonder how long it takes to wither the corn crop in order to quickly and inexpensively finish this quest. The corn crop does not wither, however. We suggest planting strawberries or watermelons and allow those to wither for quest completion."
Wikia Farm page - http://empiresandallies.wikia.com/wiki/Farm