Tanks with halides we run for 8 hour daylight cycles, with actinics coming on an hour before the halides come up and going off an hour after they go down.

PC's we generally run 11-12 hour photoperiods. I haven't really done enough experimenting with PC's to determine what is more damaging, long run times or number of starts. However fans to dissipate the tremendous heat developed at the end cap end will help improve their rather short lifespan. (best we've gotten is ~ 8 months)

I've found that the length of time a halide runs has very little effect on its life expectancy; instead the number of times it's actually fired will have a more dramtic effect. The 'probe start' is just that, there is a probe to start the arc through the envelope to start the light output, each time it's fired a bit of the material it's made from is expended, until it's too short to effectively start the from end to end.. The resulting grey to black coloring beginning at the ends of the inner envelope is what remains of the probe.