Keyhole operates on a subscription basis so the next billing date will be on the same day you subscribed. For example, if you subscribed on the 3rd of October, the next payment date will be on the 3rd of November.
You can also find this information directly from your Keyhole account, just go to Settings > Billing / Subscriptions > View Invoice:
Your data (monthly posts) also follows the billing cycle, so your data will reset on this same date.
Note: Deleting a tracker will not restore the data you have used for the month as the data has already been consumed.