It is the maximum time involved to complete an activity in a process. For any product/process planned through material requirements planning (mrp), it is found by reviewing each bill-of-material path below the item, and whichever process or route adds up to the highest number defines cumulative material lead time.
The following is an illustration showing the cumulative lead time in manufacturing firm
He above diagram shows how various assemblies take different times to get the job done.
Each assembly has a pre processing lead time, processing lead time taken by the assembly, subassembly and components, and then the post processing lead time.