We introduce relaying utilization statistical metrics to quantify the contribution of a relay node to the overall relaying process in cooperative diversity systems. Motivated by the second order statistics of the relay fading channel, the following metrics are defined: i) the average relaying selection rate, ii) the average relaying utilization time, and iii) the average relaying waiting time. These metrics are important for the cooperative systems' design, especially for applications where power consumption and delay are major concerns. As an application, we present exact analytical expressions of these parameters in an amplify-and-forward relaying system with selection diversity and diamond topology over Rayleigh fading channels.