The degree of inactivation by ultraviolet radiation is directly related to the UV dose applied. The UV dose is the product of UV intensity and exposure time. This
dose, sometimes referred to as fluence, is commonly expressed as millijoule
per square centimeter (mJ/cm2). The units "J/m2" are used in most parts of the world except for North America, where "mJ/cm2" are used.
"Even at the minimum acceptable irradiance in a biosafety cabinet of 40 W/cm2 (U.S. Department of Health and Human Services et al., 2000), it takes 12.5 minutes to reach 30,000 J/cm2 (1 W= 1 J/sec), which has been listed as germicidal for spore forming organisms by one UV manufacturer (www.uvp.com/pdf/ab-115.pdf)."
You can find the UV-C exposure dosage needed for each level of reduction (spore, bacterium, virus, molds, etc.) in the attached UV data sheet.