Well since nobody replied, I guess I should suggest the development tracker. Sometimes the boards will be marked with approximate due dates. There's also a tracker of some sort, I'm not sure how to reach it though since I've only seen it via links.
UWE generally makes a release about once a month (usually on a Thursday) - sometimes faster, sometimes slower. There is no release schedule or official timeline. If you are looking for the target date for the next standard release, your best bet is to look in trello ( https://trello.com/b/yxoJrFgP/subnautica ) at the top of the current release build. For example, it says "To Do B56 "MacWorld" (June 11th)".
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There's also a tracker of some sort, I'm not sure how to reach it though since I've only seen it via links.