Duration of the evolutionary phase spent by model stars of different metallicity in the box delimiting the J region of the (J − K, J) plane, according to the definition by Magnus et al. (2024), as a function of the stellar mass (left panel) and age (right). The masses of M <  1.5 M_⊙ stars reported in the abscissa of the left panel refer to the values at the start of the core helium-burning phase.