There are currently character copies open for players, and vendors available to purchase extra items to test that can be found in Oribos, the main city of the Shadowlands.
Players can download the Public Test Realm client and test all the recently announced features coming with patch 9.1.5, which has a lot of restrictions removed based on player feedback from the early stages of Shadowlands development.
Players can follow Blizzard's development notes thread on the forums, which is constantly being updated with all the new features added to the PTR build.
World of Warcraft Patch 9.1.5 PTR Is Now Live
The PTR phase of an x.1.5 patch typically does not last that long although they have previously not had as many new features coming to the game as this one does, we can only guess when we can see the patch reach live servers.
We typically see an x.1.5 patch released three months after the previous patch, although something of this size may take a bit longer.
Regardless, going off the three-month schedule lands a release date sometime in October, which gives over a month of PTR testing, and there is a Timewalking Week scheduled for the 19th of October, which is a very likely candidate for a potential release date.