The authors have investigated the average (mean) residence time of heterodisperse grain fractions in rolling bed of continuously operating, one and two-cascade granulator discs of different types (cone-frustum type, cylinder type). They have established that important information about the operating conditions of granulator discs can be obtained from the experimental results, and this extends the known facts about the mixing conditions measured from residence time distribution by feeding in marked material (impulse disturbance). They have determined the reduced virtual mean residence time of grain fractions as a function of several parameters. They have confirmed previous observations with the two-cascade disc, where the first and second cascade have different properties. With regard to the segregation of different grain sizes, the differences are not only in the intensity of mixing.