Practical limit to number of files which can be handled?

I've got a fairly humongous media library spread across 7 drives and for a while now I've noticed that new additions aren't being picked up unless I rescan the drive (by removing it and adding it back in).
I'm wondering if there is a practical limit to how many files can be handled before the automatic polling/updating starts to buckle?
I'm wondering if there is a practical limit to how many files can be handled before the automatic polling/updating starts to buckle?