At this juncture, most indie authors know about the Author Earnings Report - the brainchild of indie authors Hugh Howey and Data Guy. The reports dive deep into the issue of (drumroll please)...author earnings, and does an excellent job of presenting information on that subject.
In the February 2016 edition, the downloadable spreadsheets - containing the raw data on which the report was based - appear at the bottom of the report itself. That data contained the following sales rank chart (which I have since incorporated into my Kindle Sales Rank page):
Now, suppose that you, as an author, wanted your book to break into the Top 100 on Amazon. Per the chart above, you'd need to notch 994 sales in a single day (almost 1000 books!) to achieve that. Maintaining said ranking would appear to be much easier, as you'd only need 497 (just under 500) sales per day to keep that spot.
In short, it appears that selling a particular number of books will help you obtain a certain ranking (say, 1900 to reach #35), but thereafter you only need to sell half that number (950 in our example) in order to stay in that position. (Not that selling 950 books is easy by any stretch of the imagination.) All in all, I think this is a fascinating chart that - if nothing else - gives you something to shoot for. (And my personal philosophy in this situation is that you should aim high...)