The aim of this study is to propose an analytical model for assessing the well-mixing length of a tracer in a ventilation duct. The first part of the article is devoted to describe an experimental bench developed for validating the proposed model. This bench allows to follow the evolution of a tracer injected at a source point in the center of a duct by using an original optical measurement technique. In a second part, an analytical model for the spatial evolution of a tracer concentration in a circular duct is developed, taking into account an eddy viscosity model. The diffculty for applying this model to industrial cases led us to propose a simplified version that can be used for a non-dimensional distance greater than 20 diameters. The latter was then inverted in order to access to two criteria: the coeffcient of variation in the duct section and the difference between the local measured concentration and the expected homogeneous concentration. Each one has its interest depending on whether a global information on the duct section or a local information (on the axis for example) at a given distance is required.