Ratios of lifetimes of different evolutionary phases to total lifetime as a function of initial mass, for helium stars of different metallicities. Top panel: ratio of the time spent with a nitrogen-rich envelope to total lifetime. Middle panel: ratio of time spent with a carbon-rich envelope to total lifetime. Bottom panel: ratio of lifetime spent with an envelope with surface oxygen abundance higher than 0.05 after core helium burning to the total lifetime. The dashed lines in the bottom panel indicate the ratios of the lifetime spent with an envelope with surface oxygen abundance higher than 0.05 to the total lifetime, and the dot-dashed lines indicate the ratios of the lifetime after core helium depletion to the total lifetime.