Snip onion leaves for flavoring throughout the season. Harvest green onions when bulbs are no larger than the diameter of the leaves. Bunching onions can be harvested when bulbs are 1 to 2 inches in diameter; split them off from the outside of the bunch. Bulbing onions are mature when the tops turn yellow and start to fall over.
For more information on growing Onions, check out this great guide from Harvest To Table: