when will the next update be and how much content will it have
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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.