The human eye might not notice any difference in light intensity or subtle shifts in spectrum over as few as six months. Your plants will.
Our research indicates that burning lamps for 12 or more hours every day diminishes both intensity and spectrum. This happens with all lamp brands, since the chemical reactions within the arc tube (which create the light and spectrum) deteriorate over time.
Man-made lamps do wear out, and plants are quick to recognize these subtle changes. For example, think of putting an outdoor vegetable garden in a semi-shady spot. Sure, the plants may grow—but not to their optimum potential.
For fuller, thicker vegetative growth and greater yield, lamp replacement should be part of a regular maintenance schedule. Keep old working lamps for troubleshooting your system or as a temporary replacement if lamps burn out.