Harvest:
Harvest is near when ears are plump and silks have withered, about three weeks after the silks appear. To know when to pick corn–apart from ears being plump, pull back part of the husk and pierce a kernel with your thumbnail. If their milky juice spurts out, the sweet corn is ripe. (If the juice is watery, the corn is immature. If the juice is pasty, the corn is past its prime.)

For more information on growing Sweet Corn, check out this great guide from Harvest To Table:
https://harvesttotable.com/sweet_corn_growing_tips/