Timescale required to reach the ionization, thermal, and chemical equilibrium states in the diffuse medium as a function of the proton density n_H (red curve), and corresponding equilibrium values of the ionization fraction, x_e = n(e⁻)/n_H (blue curve). The equilibrium timescale is estimated as the time at which the kinetic temperature, the electronic fraction, and the chemical abundances of all species are within 10% of their equilibrium values, starting from random initial conditions.