I like one particular eye-opening chart contrasting the length of each special counsel and independent counsel investigation and the indictments and guilty pleas obtained in each:
The article was published on May 17, 2018, the date when the Mueller investigation had been in operation for one year. The Whitewater (Clinton) investigation lasted more than seven years, 77% of the time from Clinton's election win to the time he left office. Iran-Contra lasted more than six years; Watergate was just over four.
The article looks in some detail at those three previous major investigations. Here's one comparison it draws:
As the Russia investigation enters its second year, the most important variable may be how long Mueller can keep his job. Watergate, Iran-Contra and Whitewater all had one thing in common: They lasted at least four years. Given the reports that Trump has already twice considered ordering Mueller’s removal, it’s not clear that the investigation can survive that long — at least, with Mueller at the helm.