There is no answer to this as it varies very much depending on the type of soil you are talking about and the conditions on earth at the time.

In places that are very warm or very wet with fast moving waters for instance then times are quicker than places where very little happens for thousands of years at a time.

However, the process is very slow indeed and therefore, you would be looking at hundreds of years to produce an inch of soil – and 1,000 years+ to get two or more inches.