Default for IDLE worker is 15MN. check in your notification menu also that you checked IDLE WORKER. And that monitor box is checked for every of your workers.

Use the API to make your own monitoring too, That's what i do   Sometimes you get false positives (receiving idle mails when your workers are running normally) i checked this behaviour with reccurent API query, so i let email alert but i rely more on the API for monitoring.