It's just under 29 km. If you are feeling fit, and the weather is conducive to a longer day, then go for it! Remember though that there are still hills to climb after O Cebreiro.
We did it this past June however we did the climb up to O Cebrerio on horseback. Victor has 6 great horses down at the bottom that you can hire and he escorts you up to the top. You need to spend time in O Cebrerio-it is just too lovely to just pass through.
I second Via2010 opinion . Have done exactly this segment . The climb was just over 1 hour. Certainly doable
In my case, i began my Camino at SJ, so it was about 4 weeks into the trek. Fitness- i am a grandfather of 12, so you can guess my body has seen better days... weather- it actually rained that day.I agree it is doable but it all depends on the person who is doing it ie level of fitness, first day on the Camino or started in Le Puy or Geneva (or further!) etc etc
Does also depends on the weather imo.
Good luck anyway