Selected answers from the Dumb SEO Questions G+ community.
Michael Martinez: It takes as long as it takes. They crawl the Web continuously because they cannot crawl it all at once. And their crawling is probably also throttled by how fast they can process the data they collect.
Allan Khazak: Thanks for the response, tools like SEMrush, MoZ, Ahrefs they climb the search engines or where exactly do they get their data?
Paul Thompson: Each of those tools has their own crawler bot that attempts to discover and index links. Subject to the same limitations mentions above.