It takes 12 to 15 weeks for the corn ears to be ready for harvest. The tassles on top of the stalk, as referenced by YvonneM come on somewhat before that and the ends of th 'silk' on each ear after the onset of the tassles. The best way to test for harvest readiness is to peel back a little of the husks covering the corn ear and see if the cob is filled with kernels. If it is, puncture one of the kernels with your fingernail. The fluid coming out should be milky white, not clear. IAP's is correct, but this can be mitigated by taking a bucket of really cold water with you to the field and placing each ear in it (don't husk them however) and return to your kitchen as quickly as possible. Clean the ears still in the husk with cold running water to remove dirt and bugs and place in the fridge. Keep them as cold as possible without freezing and they'll be good for about 5 days. We like to pull back the husks, clean of the silk strands (there's one for each kernel), saok them in cold water for a few minutes, place a couple of pats of butter around the ear and tie up the husks again with buthcer's string and place directly on the barbeque grill. Turn occasionally for about 15 to 20 minutes... Superb!...