According to The Goldfish Tank, both goldfish should be fed tiny pinches of food for about a minute while it is being consumed, two to three times per day. In lower temperatures, goldfish eat less.
Underfeeding the goldfish is safer than overfeeding, which can be dangerous. Too much food potentially causes blocked intestines or a swim bladder disorder. If a goldfish has a trail of waste, it is eating too much food. Goldfish can live for quite a long time without feeding, and rivalry for food between goldfish does not tend to be an issue.
Food that has not been consumed after a feeding session should be removed immediately to prevent water pollution.