Safe recommended setbacks range from 30 to 40 feet to a distance equal to twice the height of the bluff. The setback should reflect the rate of erosion of the toe of the slope due to river or marine erosion and the nature of sliding likely. Also, setbacks should reflect more than just risks from landslides. Along environmentally sensitive shorelines, a larger setback may be required to protect ecology, water quality, and public resources. A landslide destroyed my beach stairs and my bulkhead.
Most insurance policies exclude damage caused by earth movement or landsliding.
Read your policy carefully. In general, trees help remove water from soils and sturdy roots help stabilize a slope. However, in the event of a slide, trees can become lethal projectiles. For more information about tree removal consult, Managing Vegetation on Coastal Slopes. I've had a slide, but can't afford a consultant.