I have found that "recently", the historical information in CDOL has become much more complete, and seems to be more consistent among all the units I have looked up. For me in CDOL, the relatively new "Historical Information" tab shows up for wards/branches, stakes, missions, and even areas. For wards, most seem to list past Bishops, 1st/2nd Counselors, Exec Sec, Clerks, and Finance Clerks. Stakes seem to have Stake Presidents, 1st/2nd Counselors, Patriarchs, Exec Sec, Clerks and Assistant Clerks. Areas have everything from the President to Medical Advisers. And I'm not aware if CDOL access is further discretized to allow certain access to certain things like Historical Information, based on callings.
Regarding the O.P., I don't see any true pattern for how far back historical data is listed. For many wards I've checked out in CDOL, many I've seen generally shows the Bishop back to the '70s (which for us in the mission field is usually back to our beginnings, or before our wards/stakes even existed
) But there appears to be a lot of variations, I've seen some legacy wards that have more data, at least for the Bishops. I'm no data historian, but it looks like around 2009 or so the Church put a lot of info into some kind of database, because that's when a lot more of historical calling info appears in different units.