Timescales required by the species to reach their equilibrium abundances as a function of the density of the gas for a temperature fixed at 50 K (left panel) and as a function of the temperature of the gas for a density of 10 cm^-3 (right). The H₂ timescales (coloured curves) are computed in configuration (a) (see main text) for f_sh,H2 = 10⁴ (solid), 10² (dotted), and 1 (dot-dashed). Each curve results from an average of timescales computed for ten random initial conditions and two values of A_V (0.1 and 1). The timescales of single molecules (coloured points) and the range of timescales of all other species (black segments) are computed in configuration (b) (see main text). These timescales are averaged over ten random initial conditions, two values of A_V (0.1 and 1), and four values of f(H₂) (10^-3, 10^-2, 10^-1, and 1).