Answer these questions:

A user logs in to the database and does nothing, just sits there in an idle session. What schema is he or she using?

A logged in user does a simply query against a table in your schema of interest and the processing time is less than one second. Is that user really "using" the schema, and how would your procedure catch that use of the schema?

I don't think your questions are vague. I think what you are trying to accomplish isn't realistic or practical, given how Oracle works.