The segregation of a granular medium in a two-dimensional rotating drum is studied. The particle assembly consists in a bi-modal distribution of disks, with a fixed size ratio of 0.6 and varying relative concentration C. The drum is half-filled with the mixture of grains and it is rotated at a constant velocity Omega so that a permanent surface flow sets in. A very rapid and quasi-complete segregation occurs where the smaller disks gather in a central core. Experimental results on the segregation kinetics are presented for various C and Omega, together with a theoretical analysis of the kinetics of this segregation process based on a description of the surface flow. The segregation can be described as a single-particle process, controlled by a differential convection of the small particles. We show that the steady state is reached exponentially fast in time with a characteristic time constant tau(s), or rotation angle Theta(s) = Omega tau(s). The segregation time tau(s) is predicted to be roughly independent of the concentration in small particles C, and of the rotation velocity Omega. These properties are confirmed experimentally.