Hi
Leeks take 6 month to mature properly.
Do not heap the dirt up too early as the young leeks are prone to disease and rot.
I have used cardboard milk/juice cartons with the end cut out around them instead of dirt to make them grow a bit longer and have a nice white stems.
I have also used 100mm PVC water pipe cut into 100-200mm length depending on the size you want this is expensive but can be used over several years where the cardboard tends to break down quickly.
Do not let them dry out but do not over water then as they are prone to rot.