Detecting if user has not done any activity on the web page using javascript

var timeout = false;
function checkActivity() {
    clearTimeout(timeout);
    timeout = setTimeout(function () { alert('inactive'); }, AmountOfTime);
}
document.addEventListener('keydown', checkActivity);
document.addEventListener('mousedown', checkActivity);
document.addEventListener('mousemove', checkActivity);
checkActivity();

What this does is run a function after AmountOfTime milliseconds. It is reset on any user activity on the document.

You can handle Keyboard Events and Mouse Events in JavaScript.

You can have a timer counting down to when the user should be shown that message, and reset the timer whenever you get an eligible event.

You can use this (library)[https://github.com/mikesherov/jquery-idletimer] to detect, if the user is idle.

Example:

$(document).on('idle.idleTimer', function() {
    //do something on idle;
    });
$(document).on('active.idleTimer', function() {
    //so something on activity;
    });
$(document).idleTimer(1000, {
    startImmediately: false,
    idle: true,
    enabled: true,
    events: 'keydown' //there are more events in the docs;
});