Based on testing, most places or devices emit lower than 30 mG. So 30 is kind of a good number to set the alarm to detect something significant. It's really up to user. The Environmental Protection Agency recommends exposure to be limited to less than 2.5 mG so you can put the alarm there. But it will almost always trigger when near electrical devices.
Just curious.... I've discovered my whole environment is an awful place to be.... I'm sure glad I'm not the excitable type :-) One interesting thing I've found is e-cigs produce very high fields and since they are in direct contact with my hand for many hours a day they could actually become an issue (I'm developing arthritis in my right hand)...