A simpler approach might be:
Create a table in the back-end db with a single field (Date/Time) and a single record. Default value of field is for example 1/1/1900. If you need to log everyone off, set this field to the appropriate time, eg Now() + half an hour.
In the front end create a form which just has a label control, a button and a timer. Default the form to visible = false. Set the timer to something like every 10 minutes. The timer event checks the field in the table you created, if the value is a future date/time, then display the form setting the label to some suitable text. The button would just be an OK button to rehide the form. Once the user has been notified adjust the timer to either repeatedly warn the user and /or to exit Access. Make the form modal (acDialog) - this will force the user to respond to it.
Hope this helps.