Neonblack510,
Welcome to Bytes!
First, I would recommend that you only do this with a compiled DB in .ACCDE format. This ensure folks can't mess with your DB or close forms they shouldn't.
Second, I have something similar to this, in that whenever any of my Users open our DB, I have a form open in hidden mode and its sole purpose is to run the timer (I think I have it set for every 60 seconds). It checks the status of the DB and whether or not I've told it to close on the user's system.
You can do something very similar to this. Open a form in hidden mode and run the timer. Have it check for the time of day, and if the time passes your threshold, then just have the DB quit, or have another form pop up that tells them to quit.
All relatively simple in concept and execution....
Hope this hepps!