River engineering projects are usually designed based on the peak discharge concept. However, some river processes depend on the shape and duration of the flow hydrograph, and their modelling is particularly relevant for designing protection and retention structures. Several authors in the literature proposed defining the hydrograph shape associated with a peak flow discharge. However, all the suggested methodologies quantify hydrograph duration by empirical formulations. In this work, we present a theoretical approach to calculate the duration of a flow event's raising and falling limbs that includes average properties derived from a statistical description of the flow regime. We consider the stochastic trajectories going from an initial flow value to the peak one and vice versa and the associated Mean First Passage Time to account for stochastic trajectories that do not reach the other boundary. The formulations are tested against data from the literature with good agreement.