Add-Type -AssemblyName System.Windows.Forms

Add-Type @"
using System;
using System.Runtime.InteropServices;
public static class IdleTime {
    [StructLayout(LayoutKind.Sequential)]
    public struct LASTINPUTINFO {
        public uint cbSize;
        public uint dwTime;
    }

    [DllImport("user32.dll")]
    public static extern bool GetLastInputInfo(ref LASTINPUTINFO plii);

    public static uint GetIdleTime() {
        LASTINPUTINFO lii = new LASTINPUTINFO();
        lii.cbSize = (uint)System.Runtime.InteropServices.Marshal.SizeOf(typeof(LASTINPUTINFO));
        GetLastInputInfo(ref lii);
        return ((uint)Environment.TickCount - lii.dwTime) / 1000; // seconds
    }
}
"@

function Start-Activity {
    [CmdletBinding(DefaultParameterSetName="Activity")]
    param(
        [Parameter(Mandatory=$true, ParameterSetName="Activity")]
        [double]$Hours,

        [Parameter(Mandatory=$true, ParameterSetName="Activity")]
        [double]$IntervalSeconds,

        [Parameter(Mandatory=$true, ParameterSetName="TestIdle", HelpMessage="Prints idle time continuously.")]
        [switch]$TestIdle
    )

    if ($PSCmdlet.ParameterSetName -eq "TestIdle") {
        Write-Host "Idle monitoring started. Press Ctrl+C to stop."
        while ($true) {
            $idleSec = [IdleTime]::GetIdleTime()
            Write-Host ("Idle time: {0} seconds" -f $idleSec) -NoNewline
            Write-Host "`r" -NoNewline
            Start-Sleep -Seconds 1
        }
    }
    else {
        Write-Host "Activity started"
        $endTime = (Get-Date).AddHours($Hours)

        while ((Get-Date) -lt $endTime) {
            [System.Windows.Forms.SendKeys]::SendWait("+")
            Start-Sleep -Milliseconds 100
            Start-Sleep -Seconds $IntervalSeconds

            $remaining = $endTime - (Get-Date)
            Write-Host ("Time left: {0:hh\:mm\:ss}" -f $remaining) -NoNewline
            Write-Host "`r" -NoNewline
        }

        Write-Host "`nActivity finished"
    }
}