The bulbs need to be changed yearly. The company that manufactured mine stated to use them longer places a risk on the ballast and it can burn out as a result. Also the spectral efficiency declines over time. The bulbs only are good for so many "lights" so avoid turning them on and off. This will wear them out sooner.
I use a plain white plastic garbage bag, one layer thick, to cut back the light where needed, and it seems to work just fine for me.