The purpose of this study is to determine the range of validity of the instantaneous recycling approximation (IRA) in the models for the chemical evolution of galaxies. To achieve this and in order to be able to follow the evolution of the gas mass and the gas metallicity with time without making the instantaneous recycling assumption, I constructed a simple numerical code. This same code was later converted to a form assuming the instantaneous recycling approximation and the two groups of solutions (with and without IRA) were compared. The results show that in the case of a star formation rate which at early times builds up to a maximum and later declines, a physically important departure from the instantaneous recycling approximation is observed in: a) The evolution of the gas mass at late times when the remaining gas mass density becomes a small fraction of the total density and b) the time evolution of the abundances in the interstellar gas of the elements contributed by the low mass stars. A less significant discrepancy between the two solutions (with and without IRA) appears at late times in the cases of the abundances of the elements whose origin are the intermediate and high mass stars.