Random vibration theory (RVT) site‐response analysis is an attractive alternative to time‐series (TS) analysis because it does not require input acceleration‐time histories. A critical part of the RVT approach is computing the root mean square (rms) of the oscillator response from the Fourier amplitude spectrum and the key to this computation is the duration of the oscillator response. Various models are available that predict the appropriate rms duration ( ) for RVT analysis. However, these models do not consider the influence of the dynamic site response on the duration of the oscillator response. Using the from these existing models to compute the RVT soil surface response spectra results in larger site amplification predictions relative to TS analysis. This article investigates the influence of site response on the duration of oscillator responses at the soil surface for use in RVT site‐response analysis. Using the results from TS analysis, an empirical model is developed that predicts the modification of due to site response as a function of various site properties and the ground‐motion duration.