This article has information on growing chives indoors: https://www.gardeningknowhow.com/edible/herbs/chives/growing-chives-indoors.htm
You can harvest leaves anytime you want once plants reach 6 inches (15 cm) tall. Cut leaves off, leaving at least 2 inches of growth above the soil. Keep in mind that once the plants bloom, the leaves will start to shrivel and turn yellow. Unless you plan to use the flowers, harvest the leaves steadily to prevent flowering.