If they return the proper 404 headers, they will be removed relatively fast, depending on how high your rank was. Higher ranked sites get checked more often. There isn't really a set timeframe for this.
If you still have access to these domain, add a hard 404 error. No nice messages (unless you get some visitors here).
For future sites I suggest a change in tactics:
We have a simple php file, that if included checks if
$_SESSION['exampleKey'] has a value. If it does not, it'll show a form with a password input in it where the user can submit a password. If the proper password is entered, set
$_SESSION['exampleKey'] = true; and show the actual page.
When you're done, stop including that file.
This way Google (and other sites) will not see anything worth while. Also, you have some protection against nosy visitors.