What I've always heard is that the UV-B emissions cease long before the visible light emissions - so it looks fine to the human eye, but is no longer putting out the most valuable wavelengths.
I replace every six months. I've been using mercury vapor bulbs for six years with no problems.
I have a meter coming soon so ill give it a check and see whats happening. Ive had it for 4 months and still have the initial readings here somewhere. Ill dig those out and see how its getting on.