Ripped off a web page.
Tube manufacturers recommended that you replace the tube at least every 12 months (sometimes sooner), as UV output decreases with time, and after 12 months the tube is no different from a standard fluorescent tube in terms of UV output. A good way to ensure that you change your bulb regularly is to write the date it was first used on the end of the tube in a permanent marker. This way it doesn't matter if you move the tube, change setups or whatever, as you'll always know when it's time to change the tube.
UV wavelengths are filtered by glass and plastic, so it is important that you have the UV bulb within the enclosure, not shining through the top or side. 95% of UVB light is filtered out by a sheet of glass. Aluminium fly screen filters out 30% of UVB light, so it's also important to have wide mesh screens over the tubes if you need to protect the animal from burns. In a lizard enclosure, you might get away with not covering the bulb, especially if the lizards you're keeping aren't good climbers. If you're housing a snake, a wide mesh screen such as 'mouse wire' is best, which should allow at least 95% of the light through.
UV light levels also decrease as you move away from the bulb's surface, so it's important that you position the bulb and your cage's furnishings so that your animals can get as close as possible to the bulb - a basking spot 300mm (12") from the bulb is ideal.
I'm still looking for the link that had a graph showing the differences between tubes,natural sunlight and natural light in shade.It could be somewhere in this link but I couldnt be stuffed searching through it
http://www.reptilesdownunder.com/reptile/enclosure/uvlighting.php