How to detect a Winforms app has been idle for certain amount of time

There are many ways to do it and the answer somewhat depends on what you need to do. You are clear and specific about what you need. The following is something I developed that probably would fit your requirements. What it is doing is using Application.Idle to determine when the application has finished processing messages then it sets a timer and filters (listens to) all messages for the application and if a relevant message (such as mouse or keyboard) is received then it resets the timer. It ignores mouse move since it is possible to move the mouse over the application without using the application. It has been a while since I wrote that so I am not sure of the details but I could figure it out if necessary. Note that this is a console program to make the sample easier to try but the code is intended for a forms application.

using System;
using System.Security.Permissions;
using System.Windows.Forms;

namespace _121414
{
    static class Program
    {
        public static Timer IdleTimer = new Timer();
        const int MinuteMicroseconds = 60000;
        static Form1 f = null;

        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            LeaveIdleMessageFilter limf = new LeaveIdleMessageFilter();
            Application.AddMessageFilter(limf);
            Application.Idle += new EventHandler(Application_Idle);
            IdleTimer.Interval = MinuteMicroseconds;    // One minute; change as needed
            IdleTimer.Tick += TimeDone;
            IdleTimer.Start();
            f = new Form1();
            Application.Run(f);
            Application.Idle -= new EventHandler(Application_Idle);
        }

        static private void Application_Idle(Object sender, EventArgs e)
        {
            if (!IdleTimer.Enabled)     // not yet idling?
                IdleTimer.Start();
        }

        static private void TimeDone(object sender, EventArgs e)
        {
            IdleTimer.Stop();   // not really necessary
            MessageBox.Show("Auto logoff");
            f.Close();
        }

    }

    [SecurityPermission(SecurityAction.LinkDemand, Flags = SecurityPermissionFlag.UnmanagedCode)]
    public class LeaveIdleMessageFilter : IMessageFilter
    {
        const int WM_NCLBUTTONDOWN = 0x00A1;
        const int WM_NCLBUTTONUP = 0x00A2;
        const int WM_NCRBUTTONDOWN = 0x00A4;
        const int WM_NCRBUTTONUP = 0x00A5;
        const int WM_NCMBUTTONDOWN = 0x00A7;
        const int WM_NCMBUTTONUP = 0x00A8;
        const int WM_NCXBUTTONDOWN = 0x00AB;
        const int WM_NCXBUTTONUP = 0x00AC;
        const int WM_KEYDOWN = 0x0100;
        const int WM_KEYUP = 0x0101;
        const int WM_MOUSEMOVE = 0x0200;
        const int WM_LBUTTONDOWN = 0x0201;
        const int WM_LBUTTONUP = 0x0202;
        const int WM_RBUTTONDOWN = 0x0204;
        const int WM_RBUTTONUP = 0x0205;
        const int WM_MBUTTONDOWN = 0x0207;
        const int WM_MBUTTONUP = 0x0208;
        const int WM_XBUTTONDOWN = 0x020B;
        const int WM_XBUTTONUP = 0x020C;

        // The Messages array must be sorted due to use of Array.BinarySearch
        static int[] Messages = new int[] {WM_NCLBUTTONDOWN,
            WM_NCLBUTTONUP, WM_NCRBUTTONDOWN, WM_NCRBUTTONUP, WM_NCMBUTTONDOWN,
            WM_NCMBUTTONUP, WM_NCXBUTTONDOWN, WM_NCXBUTTONUP, WM_KEYDOWN, WM_KEYUP,
            WM_LBUTTONDOWN, WM_LBUTTONUP, WM_RBUTTONDOWN, WM_RBUTTONUP,
            WM_MBUTTONDOWN, WM_MBUTTONUP, WM_XBUTTONDOWN, WM_XBUTTONUP};

        public bool PreFilterMessage(ref Message m)
        {
            if (m.Msg == WM_MOUSEMOVE)  // mouse move is high volume
                return false;
            if (!Program.IdleTimer.Enabled)     // idling?
                return false;           // No
            if (Array.BinarySearch(Messages, m.Msg) >= 0)
                Program.IdleTimer.Stop();
            return false;
        }
    }
}