An analytical description of devitrification kinetics induced by time-dependent nucleation and growth of crystallites during a quench-heating cycle is proposed. Relevant experimental situations include differential thermal analysis (DTA) or differential scanning calorimetry (DSC). The proposed description involves very few assumptions regarding the temperature dependence of various kinetic parameters, but rather employs measured values of growth rates. It is shown that the conventional description of nucleation based on the steady-state approximation, as a rule, is inapplicable for the description of the DTA/DSC experiment for experimentally reasonable quench/heating rates. The latter is confirmed by the analysis of available experimental data on o-terphenyl and lithium disilicate.