Blizzard is continuing their philosophy of having the expansion's first raid launch a few weeks after the expansion begins - they want people to be able to take their time leveling up, running heroics and world quests and the like. Just as Highmaul didn't come out until three weeks after Warlords launched, so too will the Emerald Nightmare take a little while before it opens up.
And kind of appropriately, it will launch on September 21st, the first day of Autumn (or sometimes it's the 22nd, but I think that's only leap years.)
Blizzard has said that they want to pace things better this time around. There was a reasonable gap between Highmaul and Blackrock Foundry, but I believe they're going to be pushing it to be a little longer this time so that they don't get ahead of themselves.
We also know that 7.1 is going to be bringing a small raid to Stormheim (whose name is likely TBD,) and this might actually launch before the official centerpiece of tier 19, the Nighthold, launches.
So I definitely would not expect to see the Nighthold come out until next year, possibly even in the spring. But we'll have seven bosses of Emerald Nightmare to tide us over, and with the strong implication that Legion will have at least three raid tiers, I think that that pacing is fine.
If we get Emerald Nightmare in September, go let's say four months before Nighthold (with the Stormheim raid dropping somewhere in the middle there,) that would mean a January release for Nighthold. Then, as it's a bigger raid, give it five or six months, meaning that tier 20 would fall somewhere in May or June (ideally.)
Obviously we won't really know how it all turns out until it happens, but I'm hoping that with a commitment to actually sticking with Legion and filling it out as an expansion, we might see plenty of smallish raids like Emerald Nightmare to bridge the gaps between the bigger "tier" raids.
With Legion conceived as a longer expansion, I really want to know how many raid tiers we'll see. The average has always been three - with Wrath's 4 and Warlords' 2 balancing each other out. But I could imagine Blizzard expanding the... expansion by adding more of these Highmaul/Emerald Nightmare/Mogu'shan Vaults-style "half-tier" raids, keeping item levels from getting utterly out of hand (though in all honesty I kind of enjoyed that by the end of Wrath my characters had stuff like over 50% crit chance.)
I'm feeling pretty optimistic about Legion in a way I definitely did not feel about Warlords. Here's hoping I'm right! (It is an even-numbered expansion, which bodes well.)