Larian Studios has updated Baldur's Gate 3's Steam page Early Access info, along with how much of the game will be available, and other information. Act 1 of the game will be available, as Act 2 and Act 3 are currently in development with no direct set release date.
This will include around 25 hours worth of gameplay. You will also have access to five Origin characters, six classes, and mostly complete functionality.
Larian Studios also mentions that they estimate that the game will be in Early Access for at least a year. Though that sounds like a long time, there are plenty of games out that have been in "Early Access" for multiple years; however, Larian Studios does clarify that you should not expect a polished game while it is in Early Access.
Hopefully, all goes well, and we can get our hands deep into playing the Early Access of Baldur's Gate 3 on September 30th!