The Full Form of LLT is‍ Long Lead Time.

Lead time in inventory management is the lapse in time between when an order is placed to replenish inventory and when the order is received. Lead time affects the amount of stock a company needs to hold at any point in time.

When considering the total amount of time for a purchase order to be delivered from a supplier, factor in the time taken for the supplier to accept and process the order. So, if the supplier only accepts reorders once a week and you place an order 4 days before the day the order will be accepted by the supplier, your inventory will need to last an addition four days. This additional 4 days is known as the reordering delay. The lead time is the sum of the supply delay, which is how long the shipment takes to reach your inventory, plus the reordering delay. 

Therefore, the lead time formula is: 

Lead time = the sum of the supply delay and the reordering delay. 

Lead time directly affects your total inventory levels. The longer your lead time the more stock you will need to hold in your inventory. Longer lead times make deliveries more unpredictable and force a company to rely heavily on demand forecasts to make orders. Once you have calculated your lead time, the next step is to employ corrective measures to reduce it.