Alright guys lets do some math. Pre-patch is a 4 week long event with a new story piece every week. Today week 3 will hit PTR for testing, so that means we have at least 2 more weeks of PTR - Prepatch. (1 extra week for players to experience the full story) Now, lets say prepatch hits 2 weeks from now, on live you would want the same allocated timeframe for players to experience this event. Now as a developer I would add another week or two for players to enjoy the prepatch event because not everyone goes on the PTR to test / experience this. So right now we are looking at 8 weeks alone.
But that said, Shadowlands would release late October / early Novemeber. Would the company want this? Because the first raid usually comes out 3-4 weeks after launch and that would put your employees to work for live-raid maintenance around Thanksgiving which Blizzard always lets employees celebrate with their families. From a company standpoint you would want your game to launch early December so your employees come back to work fully ready after the holidays beginning of January.
Now I say this, but everything is uncertain about family holidays such as Thanksgiving and Christmas due to Covid-19 and social distancing.