I have a Aqua Ultraviolet and they recommended 14 months. Did anyone check out the Aquarix software, it keeps a schedule of everything on your tank and tells you when lights, filters, CO2, and more need to be changed. Just thought I'd bring it up.
From what I understand the spectrum/intensity starts to shift (i.e., becomes less effective) sometime between 6-12 months after installation. It's closer to 6 if you run yours 24/7. Mine is off for 48 hours a week after I add bacteria to my tank (Microbe-Lift). I still replace it every 6 months.
I've always read the regular fluorescents are changed at 6 months, Power Compacts with no fan at 9-12 months, with fan 12 -14 months, UV 12 -14 months. Maybe thats in gereral or unless the manufaturer specifies other wise. The company that makes Aqua Ultraviolet also makes large units for household use on well and septics systems that can't use chlorine for bacteria, and they suggest 12 -14 months.
The bulb is never in contact with water.. the Glass tube surrounding the bulb is... so while the glass surrounding the tube is making contact with the water, the airspace surrounding the bulb gets quite hot before it can transfer heat to the glass sleeve.
I'm not sure about that, I don't think they produce much heat anyway. Mine has a 57w transformer and it's only a 8w bulb, so it wouldn't take much to cool the air gap between the quartz sleeve and bulb. Also it has a wiper feature when you use the wiper it clean any debris off of the quartz sleeve, and thats made of rubber so it couldn't take a constant high temperature.