My UV lamp is still glowing after years so i am safe...Right. That would be a beaut Tui Billboard! YEAH RIGHT

Lamps must be changed when the ballast counts down to zero - because at this time the power output has reduced to the systems rated power (when new the lamp has much more power). As the lamps power keeps reducing with use, even though it continues to glow, it will no longer keep you safe.

UV lamps dislike several things:

Being constantly turned off and on (switching)
Excessive Heat
Water on the lamp pins
Power thumps
1.  Being turned off and on

We are always asked “Why UV lamps (as in those used for whole house UV water filtration) can’t be turned on every time a tap in the house is used?

When the lamp is turned on, it takes about 5 seconds to strike and another 20 seconds to get to full operating output of UVC. In this time the pump pushes through around 30+ liters of contaminated water into the house water system. Not an ideal situation if this happens 30 to 50 times a day!

If you prefer to have the lamp only operating when needed (but only up to a maximum of once per hour), use a pump delay timer module, that holds the pump back from operating until the UV system is fully operational then allows the pump to operate. This ensures that any and all water flow is fully treated.

There is some stress placed on a fluorescent light every time it starts up.

If the lamp is installed where it is frequently switched on and off, it will age rapidly. Under extreme conditions, its lifespan can be dramatically reduced.  Each start cycle slightly erodes the electron-emitting surface of the cathodes (the heaters at each end of the lamp); when all the emission material is gone, the lamp cannot start.

This on and off is called Switching, Lamps can handle being switched up to once per hour to be able to operate to the stated lamp life of the manufacturer, However if switched, say 40 times a day, the lamp could have a life of less than 2 months.

A UV lamp does not have the usual glowing filament of an incandescent bulb, instead, contains a mercury vapor that gives off ultraviolet light when ionized.

2. Heat

When a UV or fluorescent lamp is operating 24/7 it is very important that the operating temperature is kept at an optimum temperature around the 40 to 50 degree C mark.

High powered UV lamps (say 80 watts and above) generate a lot more heat than is ideal for the system. Therefore getting rid of excess heat is an important design criteria. Choose a system that can dissipate this heat. And match the system to the daily water usage profile. High powered systems (say greater than 100w) that have no water flow for long periods, build up excess heat which damages components if the excess heat is not adequately dissipated.