Generally it's on a three month cycle, toward the end of the month. The next one will be end of this month. As I've been saying for a while though, this will be a fairly boring one. I mostly do updates to do just that: update people, and give them a sense of how progress is going and where I am in the long production timeline. Not all updates will have interesting media to share, in fact most probably won't!
I tend to keep most of the actual art assets and scenes unknown until the game actually releases, so people can experience them all at once rather than ruining it with a drip-feed. When I do share content, it's indicative of the stage I'm at: art assets (aside from scenes) are done now, so when I DO have something to share (the update after this probably) it will probably be previews of some of the new map areas, that kind of thing. I'd LIKE to keep it ALL to myself until release, but that isn't really realistic, especially considering the people supporting the ongoing dev work on Subscribe star, so this is the compromise: I share SOME, to give a sense of where things are going, and show that work is actually progressing, but keep most of it for the release.