From your sidebar dashboard, navigate Settings -> Cron Schedules.
Fill out the fields like the example, then Add Cron Schedule Here you need to add an Internal Name for the schedule you want to receive the reports. (e.g. one_per_month) and modify it in seconds for Interval (seconds) field.
After setting that, go to WP Statistics -> Settings -> Notifications tab
Enable Statistical Reports from the Statistical reporting section.
From the Schedule dropdown, choose the time you set before and Update the changes.