A "classical" probablility density function (PDF)-based approach relying on the cumulative distribution function (CDF) of the combined output signal-to-noise ratio (SNR) as well as the joint PDF of the combined output SNR and its time derivative, is used to obtain exact closed form expressions for average outage duration (AOD) of maximal-ratio combiners (MRC) over independent and identically distributed (i.i.d.) Rayleigh and Rice fading channels. On the other hand, relying on numerical techniques for inverting Laplace transforms of CDFs and for the computation of the joint characteristic function (CF) of the combined output SNR random process and its time derivative, a CF-based approach is adopted to calculate the AOD of MRC over non i.i.d. Rician diversity paths. The mathematical formalism is ilustrated by presenting some numerical results/plots showing the impact of the power delay profile and the distribution of the angle of arrivals on the AOD of diversity systems operating over typical fading channels of practical interest.