Next billing date for subscription comes in 30 days after initial payment.
If, for example, you've made payment on the 25th of July, next billing date for you will be the 25th of August.
For subscriptions with active auto-billing we add a couple of extra days after next billing date on our end so that if there appear any issues with subscription prolongation, you might still have access to the toolset till they are resolved.
Please, mind that these extra days are automatically removed if auto-billing for subscription is cancelled. In this case subscription remains active till the end of current billing cycle and account changes to Newbie afterwards.