As I require a changelog on all of my lists, I might pull the last POST line from each ... the first token would be the date of the last post.  As a rough cut, I might simply look at the dates on all of my changelog files, but subscription activity  would affect the date.

As I require an archive on all of my lists, I might look at the archives.

You might look at the system changelog.

There are lots of ways to make the estimate, but I urge you to be careful. 

• A list can be active, used and useful without having any posts. 

There are several examples of that.   For example, I have many sub-lists where the sub-lists never get a post, but are critical (to the super-list(s) to which they belong).

Cheers, Wayne