List(s) after list(s) of work

My friend has a tough manager. He says that she gives him lots of work and wants him to finish it as quickly as possible. Usually, she gives him a list of things to do. Before he finishes the list, she gives him another list. This repeats. I am going to make up two similar sentences about this below.

(1) His manager always gives him list after list of work.

(2) His manager always gives him lists after lists of work.

Do you use the singular or plural form? Please help me. Thanks.