Nighthold is a new 10 boss raid expected to launch on January 17. Assuming that all goes to plan with this date, how long should Nighthold remain the current raid content before the raid in 7.2 is released?
Given that it is a 10 boss raid, I think that 5 months is a reasonable timeframe for the raid (~2 bosses a month). This is longer than how long Blackrock Foundry was out for, but not too much longer which I think is the perfect sweet spot. This would make the next raid about a mid-July release, which means that the first tier concludes almost a year after launch.
Given that there are still 2 more raid tiers ahead and we'd already be at almost the 1 year mark by that point, I think that there is no need to drag out the raid any longer than 5 months. 1st tier = concludes July 2017, 2nd tier concludes January 2018, 3rd tier concludes August 2018.