I never use my Public Mobile data anymore (since I subscribe to cheaper mobile data elsewhere).
But I used to use it a lot more, enough to worry about it. My habit was to manually reset my software data usage counter immediately after receiving the "thank you for your payment, your plan has renewed" notification.
I suppose you could just try different data usage apps until you find one capable of understanding 30-day or user-defined billing periods.
You'll receive notifications once data usage hits 75% and 95% thresholds. Though they can be (very) wrong if you have any add-on data provisions available after plan data provisions are fully consumed.