The best use I’ve seen yet of Google Labs’ nifty new Books Ngram viewer is from Frank Chimero: “Rest in peace, medial s.” By doing a little intelligent searching on several words that would have used the long-s in earlier books but had lost that form in more recent times, he pinpointed when it changed: right around the year 1800.
Which is just about what I would have guessed, based on a thoroughly unscientific analysis of what I recall from books and publications I’ve seen from various periods. It also corresponds reasonably closely to the much more detailed summary given by James Mosley in his article “Long s,” which records not only changes in usage around the turn of the 19th century but also changes in the availability of the long-s in new type fonts.
[Image: long & short italic s, and a long-s/t ligature, from Adobe Jenson Light]