My understanding of it is that if you enter the area between 8:00PM and 4:00AM, he may spawn. So what I've been doing is fast traveling to Ustengrav, running south to Kjenstag Ruins, and repeating.
I've done this way too many times without an encounter and I'm wondering if there's a better way to get this ghost to spawn. Would reloading a save at the location or something help? Earlier, I tried to get it to spawn by simply waiting inside the ruin. Is it possible that he spawned, ran off, and disappeared while I was waiting?