Once initialized the first wave of pandemic due to Covid-19 pandemic, it was seen in most countries a common pattern that gives account about the number of infections. Nevertheless one finds also that in mosly of these datasets could have had errors acquired in the inhomogeneous mechanisms of measurement. In this paper from a general trapezoid-like representation of pandemic, the triangular part of it, is used to estimate the duration of a third wave. From this proposal both entropy and probability haved emerged in a spontaneous manner. Therefore the present theory suggests that from the peak of infections as well the duration of second wave, the duration of third wave might be calculated. Simulations from the presented theory are done and the results are discussed.