when will the next update be and how much content will it have
I'm going to assume you're asking about the Demo version and not the WIP builds from my SubscribeStar.
In that case, the current plan is to release a couple more major patches for the WIP builds before taking a large chunk of that content from those builds and bringing it to the demo version. All in all, it should add at least a whole new region with several new delves to complete.