If you want to find out how long it has been since the current user no longer made any work (=using the mouse or keyboard), you can use the following code:

using System;
using System.Runtime.InteropServices;
using System.Threading;

namespace ConsoleAppUserIdle
{
    internal struct LastInput
    {
        public uint cbSize;
        public uint dwTime;
    }

    public class Win32
    {
        [DllImport("User32.dll")] private static extern bool GetLastInputInfo(ref LastInput plii);
        [DllImport("Kernel32.dll")] private static extern uint GetLastError();

        public static uint GetIdleTime()
        {
            LastInput lastinputinfout = new LastInput();
            lastinputinfout.cbSize = (uint)Marshal.SizeOf(lastinputinfout);
            GetLastInputInfo(ref lastinputinfout);
            return ((uint)Environment.TickCount - lastinputinfout.dwTime);
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            LastInput lastInPut = new LastInput();
            lastInPut.cbSize = (uint)Marshal.SizeOf(lastInPut);
            uint idleTime;

            while (true)
            {
                Thread.Sleep(100);
                Console.Clear();

                idleTime = Win32.GetIdleTime();
                Console.WriteLine("Last user input: {0} ms ago.", idleTime);

                if (idleTime > 10 * 1000)
                    Environment.Exit(0);
            }
        }
    }

}

This example shows every 100 milliseconds how long the current loged in user has not moved/clicked the mouse or entered a key on the keyboard. If no entry has been registered within 10 seconds, the program simply closes.