I decided to not run CCleaner for a few days and see how much junk would pile up.
Let it go for a couple of days and it seems to be stuck around 160MB with my normal amount of surfing the net.
Is this common or is there a limit or something that would make it not see anymore removable junk?
AFAIK there is no limit, maybe you are visiting the same sites etc, so the amount isn't really changing as you already have those files.
Only major files like installers, downloads etc would bump it up, everything else is usually small files like logs, or histories which are in the small kb's.