Updated versions of the sd card images are on what Jools calls a “when I feel like it” basis. Whenever there’s a feeling that something’s changed enough to want to push out a new SD card image, there will be a new image.
As for the updates that you get through the update script, there are no set schedules for those updates. They happen as they get committed to the RetroPie-Setup repository. Update it whenever you get the itch.
Questions like “is it worth x?” are always hard to answer by definition. But yes, the updates are worthwhile and fix or add new stuff all the time. It doesn’t take long to run even a full binary update. Just run it whenever you feel like.