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I dont think there is a hard and fast rule for this. I have noticed ranking increases and page traffic after building out additional directories within a site. It took around a week or so to get indexed and begin ranking within the serps and drive traffic back to the site. This was a site with a modest domain authority. I think it depends on a variety of factors. One thing id recommend is track all the keywords you hope to rank for and keep an eye on the indexation of your site.