It depends on which variety you have. It should say on the seed packet how many days to maturity. Usually once they reach 6 feet.

At 4-5ft you should be able to see the tassels down in the whorl. It should tassel soon.

My corn are only two feet tall and I can see tassels down in the whorl...I don't know why.

ed:

Corn that tassels smaller than normal means the growing conditions weren't good enough to grow a big plant. Corn needs lots of water and fertility, mainly nitrogen, to grow well. Small plant equals small ears.

Looked in the whorl and i saw some tassels.