[Exim] which kind of maildrop?

hello,

i use exim with qpopper with an virtual setup
(/var/spool/domain/user).
i read in the docs that you should split up mailboxes in a way like
"/var/spool/d/domain/u/user" but i didnt find at which amount of
maildropfiles per directory you should do this.
i currently have about 1000 virtual domains in one directory.
when shoud i think of splitting them up?


robert