Most UV C lamps for commercial HVAC systems or disinfection towers are modified T5 flourescent lamps. The modifications are significant, but that still doesn't cahnge a lot of the questions about how long you can go between changes. Most T5s are rated to last 20,000 hours and their UV counterparts are typically half of that - depending on how "end of life" is determined.
Understanding T5 vs UV-C: It can't be stressed enough that even though UV-C lamps look like, operate like, and some even use regular T5 ballasts, they are different creatures. Some of the differences are small, like the UV lamps don't include phosphor, however some of the differences are major like the type of glass used. Normal lamps use an impure glass, this is partly because it is cheaper and a high-purity glass isn't needed to still allow light through and partly becasue traditional flourescant lamps produce the C band of UV light but the impure lamps keep it from being emitted. Replacement UV-C lamps use either Quartz or Sodium-barium Silicate high-purity glass so the C Band of UV light can be emitted. This is why even if the HVAC section containing UV has viewports/portholes the glass doens't need to be coated - it is such an impure product it naturally prevents the harmful C-band light from escaping the HVAC system.
But It's Still Lit: Again, these aren't for the purpose of emitting visable light, they will produce for visable light for years like normal lamps. The UV output however drops off much faster than the visable light, which means these lamps will need to be changed more often than normal flourescants to amintain their efficiency in your system. Different manufacturers will have a different acceptable level of output that they define as end of life. Most manufacturers will say 9,000 hours (or 1 year of 24x7 use) as it's been shown that the UV output will drop by 15% while some manufacturers to promote a longer life bulb will say they have a 2-year lamp. All that's happening is at the end of two years the output drops by about 50% so you are running the system at a lower efficinecy in year two of those "2-year" lamps.
Or of course, feel free to contact us with specific comments or questions, we're here to help!