They usually say "knee high by 4th of July" and you're good to go.. pretty much everything around me, including late planted corn, is at least knee high and some of it looks to be chest-shoulder high already. There are different maturity dates for corn which farmers can plant by. As long as those dates were followed, I don't see harvest being any more prolonged than normal unless we have sub-par weather during harvest season. That being said, I like hunting edges of standing corn. It seems the deer like to move a little earlier with the added "cover".