I don't remember it being necessarily time consuming (but it was very labor intensive). I would estimate less than 30 minutes--depending on what tool they used.
That said, my experience in cutting the trail would have been very different from who ever made the clearing. I was cutting a narrow trail through brush that was 3 or 4 feet high with a machette that was too long for such purposes.
There is absolutely no way to see that clearing from the roadway, let alone see inside of it. That clearing was the last thing I was expecting to see when I checked on my trail. So, yes, a person or persons inside that clearing would have complete privacy.
In fact, a person couldn't see inside the clearing from anywhere on the property except for at the trail head or on the trail. That clearing is so completely private and hidden that it really makes my mind race as to what all was done there.