I think it partially depends on how many comics are up, and the comic's overall popularity as well. Comics with fewer than 25-30 strips and low pop would be removed more quickly then some longer lasting ones, I would imagine.
I think so far they were cleaning only sites with no comics at all. In future, they might be cleaning sites with 1 or 2 comics in whole. Sites with entire archives will most probably remain untouched unless artist decides to take down archives (which actually happened quite a few times, always followed by a rage of readers).