This is just a guess but I would imagine they will drip feed content they need testing by the masses until release. I would expect to see some new area's, maybe all the classes and subclasses before launch although I doubt we will get the full game before launch. I am actually doubting we will see all of act 2 if at all as they will have internal testers doing most of the heavy lifting. Larian are not a small studio any more.

With some big titles coming out soon, they will see quite a dip in their player base so hopefully we will get something before then if they want to keep their EA players engaged..
If not this has been a great pre-order lol.

Either way I am optimistic.