Depending on the intensity of light you have and the wattage is the bulb, 7 mins is way too long. If your bulb is very close to the mask 10 seconds is more than enough.
UV-C is basically entirely blocked by the ozone layer and doesn't reach the surface of the earth.
Its extremely dangerous, and can cause eye and skin damage with under 60 seconds of exposure, it is absolutely something you do not want to be anywhere around when active.
Alternatively, would leaving things under the sun have a similar effect and, if so, how long would it take?