Well, in the past its typically 4-5 weeks from Launch til Raid release. Not so in the case of Shadowlands.
You get 2 weeks and then raids release as follow.
RAID UNLOCK SCHEDULE (NORTH AMERICA):
November 10 – Normal and Heroic difficulties
November 17 – Mythic Castle Nathria, Raid Finder Wing 1
December 1 – Raid Finder Wing 2
December 15 –Raid Finder Wing 3
January 5 – Raid Finder Wing 4
SHADOWLANDS SEASON 1
Once the gates of Castle Nathria open on November 10, Shadowlands Season 1 will also begin which introduces a new seasonal affix in Mythic Keystone dungeons along with a new season of PvP action.