You can check the valve once a month, but there is nothing to stop a toothpick, or anything else, from lodging on the seat IMMEDIATELY after you close it up again. That is the "deficiency" of a back water valve. All you can hope for is that it slows the flooding down until the city sewer can operate normally again. A 4" ball valve" can be very difficult to operate after a couple of years. A "shear gate valve", (many are combination in with a back water valve), designed for a sewer system is a better option, assuming you are home when the flooding occurs.