I'm asking because mine is strangely short and I definitely didn't clear them. However I tried changing my username (because I got some errors which I thought were due to my hostname; tried to view the history to see how I did that to change it back) and I'm not sure if that could have caused the history to be cleared?
I'm running Debian 9.1 with KDE and $HISTSIZE variable is set to its default value.
(In the root history there are also 2 commands which I can't remember having executed:
filemanager /var/mail/root and
texteditor /etc/nsswitch.conf for which I'd like to view diffs using etckeeper.)