Harvest:
Leeks can be harvested as soon as they are big enough to use. Leeks can be harvested when they are shallot-size. Leeks will be near their mature size when stems reach 1 to 2 inches in diameter and leaves are 12 to 18 inches tall.

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