The impact duration between two structures is not easy to forecast. Indeed it depends on many parameters including local parameters as well as structural parameters which characterize the dynamics of the structures.In this paper we will show that it is possible to have a prior estimation of the impact duration. In fact the simulations show that this parameter is connected to some characteristics of a specific model, the so-called antioscillator model, even for the complex case of the impact between two flexible structures. All the simulations carried out on different cases show the close relationship between the first impact duration and the antioscillator characteristics. However some limitations exist and have been highlighted.