Is it cost effective to keep turning a fluorescent tube light on and off or should it just be left on?

It is generally more cost-effective to turn fluorescent tube lights off when they are not in use, especially if they will be off for 15 minutes or more. Frequent on-and-off cycling can reduce their lifespan, but the energy savings from turning them off typically outweigh the costs of reduced longevity. However, for very short intervals, it might be more efficient to leave them on. Ultimately, the decision should consider both energy costs and replacement expenses.