The life of a 48-inch fluorescent light bulb, technically called a fluorescent "lamp," depends upon its wattage, design and application. A typical 32-watt, 48-inch fluorescent lamp will burn for about 20,000 hours, if normally left on for about three hours at a time. The same lamp, left on 24 hours a day, will last about 34,000 hours.
Average Burn Time
While, a 48-inch fluorescent lamp will, on average, burn for 20,000 to 34,000 hours, the quality of light output gradually decreases over time. The "economic life" of a fluorescent lamp is only about 60 percent that of its actual burn time. Once the economic life on a lamp expires, it grows dimmer and loses color, and may be considered useless, depending on its application.
Though the quality of a fluorescent lamp degrades during the last half of its life, it usually does so at a slow rate. The actual burn time of a 48-inch fluorescent lamp is about 20,000 to 34,000 hours, depending on how long you typically leave it on.