Many Web analysts evaluate their data on a monthly basis, but 30 days is too long an interval for monitoring key metrics.
In Google Analytics, you can set up daily or weekly Custom Intelligence Alerts for visits, conversion rate, and any other critical performance indicators. (You can also set up monthly alerts, but that is not our current focus.)
With filters and advanced segments, you can apply your alerts to very specific portions of your audience.
Remember: you do not receive email alerts for the automatic, built-in daily, weekly, or monthly Intelligence Events. To receive emails, you must set up your own custom alerts.
As a related post, see Set Up an Error Page Alert.