Usually at patches. So you will likely just need to see when the patch hits itself if it changed.

They originally let people world visit for the previous version of this (Ishgard Restoration), but everyone visited one world preventing the natives of that world from participating, and it was unfair to them so they restricted it.

Since there is a similar competition happening this time apparently, it makes sense to restrict it again, but they shouldn't keep the restriction when the competition aspects aren't happening. The track record so far though is even Ishgard Restoration is still restricted years after the FOMO is over.