Given the length of the Suramar questlines, I would expect it to take you around 6 – 8 hours to unlock Nightborne from scratch. However, it could take much longer if you need to level a Horde character to level 45 to start the recruitment quest. Additionally, you'll also have to deal with the roadblock of completing 10 world quests which will take you at least two days. So, unfortunately, you won't be able to pound out the entire chain in one sitting.