During the last regular webmaster meeting, Google Search Quality Specialist John Mueller shared a small insider. It turns out that a block in the form of additional links to a site in a snippet may appear within a month.
For example, like:
John argues that this is because Google needs to make sure that the pages that the search engine will link to will be available for a long time to come. Unlike other types of markup, which Google crawls multiple times before showing up in SERPs, it takes longer to validate the quick link block. Mueller also reminds us that adding markup does not guarantee that extended snippets will appear in the search.
John Mueller’s answer can be seen below: