Classification of fine soil is basically depends on sedimentation process of silt and clay, which consists of different particle size. Silt is bigger compared to clay, so it will settle faster than clay. Head (2006) provides approximation on terminal velocity for each particle size, based on the so called Stokes’ Law.

Particle	Terminal velocity [mm/s]	Approximate diameter [µm]
Coarse silt	1	35
Medium silt	0.1	12
Fine silt	0.01	3.5
Clay	0.001	1.2

From the provided value, the time required for the particles to settle for certain depth can be estimated using usual relationship between velocity and depth.

References:

K.H.Head (2006). Manual of Soil Laboratory Testing, Volume 1 Soil Classification and Compaction Tests, 3rd Edition: CRC Press