This sounds very doable, though only interesting for only a few people. Not really worth a new feature for everyone imo.
This should be quite easily done in an extension though. I have no experience with VLC extensions, though I downloaded an example one and it's just a bunch of Lua files. You can write your own bit of code to record the time using:
https://www.lua.org/pil/22.1.html and you can write that data to a file with
https://www.lua.org/pil/21.1.html
Upon requesting the time read you could read the file using the same functions and do the math on how much is used. In order to store the time you could use your own format or something like JSON to note the different times of use, perhaps with a category and filename.
I realize I'm asking you to learn Lua and just make your own extensions... Though that is a good idea! Learn Lua, make your own extension and learn a skill which is going to be and already is very useful