Total lifetime of helium stars, from the start of helium core burning to core collapse, as a function of initial helium star mass. The different coloured lines represent different metallicities. The downward pointing triangles at the top indicate the lowest initial helium star mass needed for a model to transition into the WC stage before the end of its evolution. The dots on each line indicate the minimum mass at which stripped-envelope stars may be classified as WN stars, obtained via Eq. (7), and using the mass-luminosity relation of Langer (1989). Helium stars below this limit do not have optically thick WR winds, and have overestimated mass loss rates.