The number of published paper per year is a good indicator. In the year 2006 around 1 million academic papers were published, in the year 2017 around 2 million per year. The cumulative sum is estimated with around 60 million papers right now. The papers can be divided into subjects like math and physics so it is possible to tell exactly how much knowledge a society has about a topic.
To draw a connection to AI it is possible to filter out only papers which are about Artificial intelligence. That means, only knowledge which is stored in a PDF file can be seen as real knowledge. For example, about the topic AGI, all the scientists have published in the year 2018 around 257 papers, according to Google Scholar. They know only what it is written in these documents. That means, if you meet one of the scholars in real life, he can only answer questions about his published paper, not more. The progress is not exponential like in the Moore's law. Every year the increase in the output is around 30%.