The dev follows a pretty strict schedule, every week they make a blog post, which can range in topic, sometimes not being about the game at all, sometimes containing teasers like the one I posted earlier. These blog posts appear on the 7th, 14th, 21st and 28th of each month. On the last day of the month, the Dev makes a post on their Ci-en, summing up the work done that month, and the contents of the new demo. That Ci-en post is where you will find the demo, so your answer is the last day of the month.
For those that liked the mecha-tentacle grab, there a complete set of five .gif covering the full content of the grab on their Nijie account:
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