It's a difficult question to answer because it's an evolutionary process. As you're using the web search, we're not trying to serve results to you that deal with your needs in the past. We're trying to serve results to you that deal with what you're looking for right now. However, we can get insight over time into what you're interested in, what you're looking for, and which results you did not find useful.
There are elements of all of that data that don't necessarily pertain to you as an individual, but that we find useful in developing new products and new tools. The distinction you made originally between personal information and other data that could be associated with your activities, but isn't associated with you, is a useful one for us.
I would say that at some point we do not retain all the data that we collect, simply because it's not useful to us. At the point where it no longer becomes useful, we delete it. We're not in the business of creating a very large bucket of information about you. We're in the business of providing very useful services and products to you as an individual.