The most important factor to consider is the wavelength of light
emitted by the Hg lamp. The most germicidal wavelength is 260 nm.
With that said, I would expect that several hours would be sufficient.
However, I would check sterility by placing an open petri dish of
bacterial agar in the box following sterilization, and see if anything
By the way, UV light is the method of choice for sterilizing air.