The Test Director forum is typically used for the older non-Quality Center version of the application.
Crossposting is a nasty internet habit where you post the same question in multiple places, the reason this is discouraged is it means there are two places the same conversation is taking place. For the volunteers here, it means we aren't sure which one to post in if dual conversations are taking place, and leads to (for the most part) the inclination to just not try to help.
As for you question:
Chapter 7 of the Administration guide will tell you the information you can pull as related to site usage.
The Site usage analysis will be able to tell you what you are after at a license level. It will report on the number of licenses which have been active over the hours of the day, and when no licenses were being used.
Insanity: doing the same thing over and over again and expecting different results
I have not researched the cleanup procedure, but would suspect you are correct. Otherwise, that table would become huge.
A quick run on a company's table shows 89,841 rows in the SESSIONS_HISTORY table. The earliest record is from Feb 2, 2007. It appear the table may keep approx 1 year of records, but is probably configurable through a Site Admin Config Parameter to adjust the retention period.