Corn on the cob takes around 5 minutes to fully cook after the water starts boiling in the pot.
So once the water begins boiling time the corn on the cob cooking time for 5 minutes and it should be fully cooked by then.
Fully cooked corn on the cob will dry fast when it's lifted out of the boiling water.
Corn on the cob is fully cooked when it's very hot and steams dry within a few seconds of removing it from the boiling water.