You'll be charged your subscription fee as soon as your account becomes fully operational or you change your pricing plan. You'll be charged on the same day of the next month.
Your allowances will reset monthly on the same day of the month (note that there can be up to 24 hours lag between subscription fee charged and plan allowances reset).
If you choose to change your plan mid-billing cycle, your subscription will start again and your allowances will be reset according to your new plan immediately.
To see your current plan, billing date, allowances and usage, click here.