A new time-optimal rheometry technique is presented. It consists in applying to the material a continuous exponential frequency sweep and analyzing its response by means of Fourier transforms. The properties of this method are that it takes the least possible time to perform the linear viscoelastic measurement in a given frequency range, and also presents an optimal signal to noise ratio in Fourier space. After validation against classical methods, it is used to characterize the gelation of a dental alginate. Properly time-resolved measurements of the frequency dependent viscoelastic modulii are presented. This allows to evaluate rigorously the gel point, using the Winter and Chambon criterion. Analysis of the data shows that a fractal networks forms inside the material after a lag time of about 350s, with a subsequent fast evolution (less than two minutes) towards the final structure.