25% of google searches are unique. I bet another 25% would be so rare and numerous you couldn't possibly have time to tune them. In fact, when you think about how much revenue a single search brings, a search is going to have to happen on the order of 1,000 times (at least?) before you can have a human hand tuning them.
That's from 2007. I wonder how much Google's autocomplete has changed that statistic. Even still, 25% of searches may be unique strings, but I doubt 25% of searches are meaningfully/semantically unique.
