Significant results concerning very low frequency dielectric relaxation are obtained by transforming the measured time data into the frequency domain. The key point of this transformation is the relation between the time range and the frequency range. An accepted relation is that proposed by Hamon, for the case when the experimental data can be approximated by the Curie-von Schweidler function, although there are disagreements between the theory and the calculated results. The theory and the experimental results are in good agreement if the isothermal current is approximated by an exponential decay function or by the Kohlrausch-Williams-Watts function. A complete analysis of the data can be performed only if the isothermal current can be approximated with one or a sum of two exponential decay functions and a new method for evaluating experimental data is proposed. The new method allows the separation of relaxation processes and the data can be analyzed using a simple model and/or the Fourier transform. The experimental data obtained for polyethylene terephthalate are analyzed and two relaxation processes at very low frequency are shown. The results are in good agreement with those obtained from alternating current or from thermally stimulated discharge current measurements. By using the analytical expression of the current and the Fourier transform the errors determined by the fact that any experiment will cover a finite range of time are estimated. In order to obtain a correct characterization of the relaxation processes the measuring range of time has to be strictly correlated with the relaxation time of the processes.