Relic neutrinos decoupled from matter around the time of nucleogenesis so had the same temperature as matter and photons at that time. Photons decoupled much later, after electron/positron annihilation which heated the photons slightly so thereafter neutrino temperature should be (4/11)^(1/3) times that of the photons. The CMBR is at 2.725K so the neutrinos should be at 1.945K or equivalently 1.676*10^-4eV. See pages 14 & 15 of: http://darkuniverse.uni-hd.de/pub/Main/WinterSchool08Slides/CosmologicalNeutrinos.pdf That temperature scales as (1+z). The latest estimate of neutrino mass is 0.32eV for the sum of the three flavours or 0.11eV each. http://arxiv.org/abs/1308.5870v2 That suggests their energy would be comparable to their mass at z~635 which is around 910k years. Obviously the change would be slow but am I right in thinking that time would be roughly when the neutrinos transitioned from relativistic speeds if the estimated masses are correct?