Maybe you can try it once a month to see how your members react. If many of them are very interested in waiting for a surprise from you, you can change it to once every two weeks or even once a week.

Don't forget to give or release surprises at certain times and announce them in your thread and your social media so that people will continue to wait for your updates faithfully. It is a very common marketing strategy used by a website to attract customers and you can also apply it to your site by making a few adjustments.