The dependencies of electrical-breakdown time delays t_d on the afterglow period tau were measured and the dependence of the mean values t_d on tau , using the so-called memory curves t_d=f( tau ), was established in nitrogen-filled diodes at 1.3 mbar and 13.3 mbar. The corresponding time-dependent variations of metastable concentration n, i.e. the curves n=F( tau ), were derived, where, at afterglow periods exceeding 1 s, the metastable molecules were predominantly in the N₂(A³ Sigma _u⁺) state. A physical model of metastable de-excitation processes has been proposed and, by solving its differential equations, the following dependencies and constants were obtained: (1) the theoretical dependence of metastable concentration on the afterglow time-the curve n=F( tau ), (2) the range of n₀, the initial concentrations, which are also saturation concentrations of metastables reached in discharges, (3) the metastable concentration-independent constant k in the equation t_d=k/n, (4) the dependence of the half de-excitation time T_1/2 on the afterglow period, T_1/2= phi ( tau ), and (5) the dependence of Z on the afterglow period, Z= Phi ( tau ), where Z is the concentration dependent ratio between the de-excitation rate via binary metastable-metastable collisions and the total rate via all other considered de-excitation mechanisms.