You find the next billing date under the Subscription menu point (https://app.storrito.com/#/subscribe). The 'Refill date' shown in the screenshot below will also be the next billing date. On this day you story posts will be refilled to the amount provided by your subscription. If you bought the Standard plan for example, then it will be refilled to 100 story posts.
Updated on: 09 / 08 / 2021