>>Is there a maximum length a line can be?
Yes, depends on the lowest freq. your shooting for and the c-t-c spacing of the drivers.>>At what number of WL do we reach diminishing returns on any added length?
Again it depends the freq.
Attached is a link to Jim Griffin's white paper on the subject, a vary good paper with lots of info, it's what I used when I designed up my arrays. Plug in some numbers and it'll give you some idea.
HTH
Bill W.