As to a clear metric, yes it depends. Rules of thumb need to be carefully evaluated if they match the situation.

One thing that I think we miss in the discussion is that, from a regulatory standpoint, a low save is (again depending on the situation) a potential emergency.

In the US, §91.119 has a very clear metric on this. Unless you are INTENDING to land (or takeoff), you must remain at least 1,000 feet above any obstacle with 2,000 feet horizontal of your position in a congested area. In uncongested areas you must remain 500 feet above the surface, except over "sparsely populated areas" and water surfaces, at least 500 feet away from, essentially, persons or man-made objects.

Sorry I am quoting regulation here, but certainly something as food for thought, and if anything a guide for serial low-savers.