Harvest:
Cut individual leaves for use when the plant is 8 to 10 inches high; cut the outside leaves first. If you harvest the entire plant, cut 2 inches above the soil, and the plant will sprout new leaves in 1 to 2 weeks.

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