Help with adding a progress bar to my compare-object script

I have a simple compare object script that I would like to add some kind of intelligent progress bar or countdown timer to.  The script takes anywhere from 1-12 hours to run depending on the folder sizes.  Here is the script:

$internal = Get-ChildItem -Recurse -path C:\Folder1

$external = Get-ChildItem -Recurse -path C:\Folder2

Compare-Object -ReferenceObject $internal -DifferenceObject $external | Out-File C:\compare_list.txt

I have a progress bar script that I found but I do not know how to implement it into my existing script.  Here it is:

for ($i = 1; $i -le 100; $i++) {Write-Progress -Activity 'counting' -Status "

$i percent" -PercentComplete $i ; sleep -Milliseconds 20}

Any help is much appreciated.