Warren,
After talking with our department mechanic who was involved in one of the seminars regarding this, he stated that the "on time" had been an evolutionary process based on the frequency of use by each particular apparatus. But, it sounds like a couple of hours per day was being used fairly frequently. As for our boats, most of them sit for many days without use, like some fire trucks in remote fire stations and would take some monitoring over several months to find what timing cycle would work best.
Again, good luck.
Doug
[/i]Lil' Brother