Map of disc lifetime as functions of stellar mass and ambient UV field strength, this latter being the main driver of external photoevaporation according to the surrogate model described in Sect. 4.2. Internal photoevaporation is set to its minimum value (L_X = 1 × 10²⁸erg s⁻¹). Other parameters were selected as M_G/M_⋆ = 0.1, ƒ_D/G = 0.0149, β = 0.9, P_in = 10 d, r₁ according to Eq. (9), α = 1 × 10⁻³, υ_frag = 2 m s⁻¹, ℰ = 1 × 10⁻³, and ζ = 1. The solid black line shows the location of a 3 Myr lifetime (a typical value). The results are discussed in Sect. 4.3.2.