420kg of weight and a drop height of say 1.5m has an energy of 420x9.8x1.5 = 6174 joules.
when your setup draws 3.5 watts or 3.5 joules/s it will run 1764 seconds or less than half an hour,
in practice less because there is also loss in the gear and generator. and then you have to pull
up the weight again...