Found this on the Foster and Smith website:
How often should I replace the bulbs and why?
The intensity of a bulb diminishes over time. Even though the bulb will still light up, it may not be emitting the necessary spectrum or intensity to support healthy growth. In many instances, the diminished light output can encourage nuisance algae growth.
Incandescent Bulbs - Replace every 2 to 4 months.
Standard Fluorescent Bulbs - Replace every 6 to 18 months depending on ballast type.
VHO Fluorescent Bulbs - Replace every 4 to 18 months depending on ballast type.
T-5 HO Fluorescent Bulbs - Replace every 16 to 24 months.
Compact Fluorescent Bulbs - Replace every 14 to 24 months.
Metal Halide Bulbs - Replace every 6 to 18 months.
Seems like quite a large time spread to me.