Great discussion item, there is no one answer but if you do some research on "Deposition of Soil" you will see statements like : One inch in 500 years and one foot in 150,000 years.
I have a geology book somewhere, if I can find it I will see what it says if anything. There are a lot of myths amongst the metal detecting community on why coins are at what depths they are.
I will try and find some factual statements about that if I can. I will say this. The amount of topsoil that forms is without a doubt different from one location to another, since it does depend on how much vegetation grows and is deposited. But a lot of factors happen before that humus turns into soil. I tend to believe in the woods the amount of deposition varies greatly, due to factors like , wind, heavy rains, forest fires, (which is a major factor), frost and I am sure there are many others.
I live where the soil is almost non-existant in the woods and also most areas experienced severe logging many times over the past two hundred years. In the woods, the vast majority of my Colonial era coins that in theory have been in the ground for over 200-250 years are usually between 4-8 inches with most closer to 4 than 8 inches.
Of course it is hard to say if the wooded sites I hunt were gardens where the soil was turned over, or heavy foot or horse traffic that would also effect the depth of the object.
Again, interesting subject, gee , maybe "sinking" of coins can be discussed also.
Don