That's an interesting idea. Right now, it's a sliding window of the last 30 days, which isn't useless, but could be more helpful if it could be lined up with the billing cycle. The thing is, different people may have different billing dates, so you can't just reset it at the start of the month for everyone, either. That means making it configurable for each customer. Not impossible, certainly, but it would require some engineering. If you'd like to see this considered, the best way to let them know is to use the Feedback feature in the Google Home app.