That matches what I saw on my Ram with Gen 3 EcoDiesel. On my Scan Gauge, I saw that 650F is where passive regeneration started to become noticeable as my Mass Soot Load values would then start to fall. Below 650F it seemed that passive regen couldn't really keep up.
On my new Jeep pickup with the same powertrain, passive regen seems to happen more efficiently, probably because underhood temps are higher due to the tighter packaging.
However, I also monitor active regen status with my Scan Guage and I rarely ever see the truck do a regen cycle. When it does it's very quick and efficient. Doesn't take long at all.