If a particle is settling in a gravitational field, it rapidly reaches its terminal velocity. In practice, the concentrations of suspensions used in industry will usually be high enough for there to be significant interaction between the particles. This could mean that the drag force is effectively increased thus slowing the settling, or particles join together effectively increasing the size and thus the settling.
The video below provides a general introduction to settling and settling curves.
The graph below shows the change in concentration of the particles with time during a batch settling process. Gypsum is a standard material with moderate hindered settling, Glass spheres are ideal and only undergo type I settling, and component A is a material with high hindered settling.