BOOM TISH!!!
I think it happens more often than every fortnight. The guys on my shift rotation seem to do it pretty much every week. I can only imagine the other 3 crews are much the same.
Tower does notify aircraft that it's a training exercise. I don't know what happens inside the cabin as to whether the passengers are informed.
I think they try to coordinate it for when there aren't going to be aircraft flying through the thick of it.