How long will it take a 100gram rock to turn to sand/dust by the elemants of nature?

Answer 1

It would depend on which agent of erosion (ice, wind, water) it is exposed to and how much of that exposure it gets. It would also depend on the rock, how resistant or non-resistant it is to weathering. All rocks are not created equal.

In other words, there is no definite answer, because there are many many possibilities for the conditions.