Properly stored corn on the cob will last up to 3 days in the refrigerator before going bad.
When corn on the cob is going bad the silk at the top of the cob begins turning brown and becomes slimy and the husks begin to turn yellow and dry at the tips.
Corn on the cob is still safe at that point but should be thrown out after 3 days in the refrigerator.
If the corn on the cob is really slimy and or has a bad smell to it then throw it out as it's no longer safe to eat.
Corn on the cob needs to be frozen to be kept longer than 3 days but after 3 days in the refrigerator or sitting out then the corn on the cob has likely spoiled and shouldn't be eaten.