My educated guess based on experience working with people affected by dissimilar causes but similar appearance to Google: a massive hit to rankings/traffic within about 48 hours, and recovery to a fraction of normal several weeks later.
I doubt that one can easily convince a judge that loss of Google rankings is a cognizable harm under tort law, but given that an agency of the US government just called you a child pornographer on your own property, if there isn't a sovereign immunity defense you will hardly need to justify what a high ranking on Google is worth to receive damages.