Harvest:
Cut cabbage when the heads are firm and the base of the head is 4 to 10 inches across. If you want additional heads from the same plant, cut the head at the center of the stem but leave several leaves attached to the stem stump. Small heads—about the size of a baseball–will grow from the stalks for later harvest.

For more information on growing Cabbage, check out this great guide from Harvest To Table:
https://harvesttotable.com/how_to_grow_cabbage/