We use water trucks for dust control on unpaved waste product haul roads at electric generating stations. Water quantity varies primarily with ambient temperature and humidity. The number of waste product haul truck cycles is of lesser importance. In (hot) summer conditions water must be applied continuously, in (mild) winter conditions, maybe a few times per day. No single answer.

www.SlideRuleEra.net
www.VacuumTubeEra.net

As SRE said it depends on a number of factors. I worked at a landfill expansion site some years ago; in the summer the water truck was driving the site from starting time to quitting time; less frequent in the spring and fall. Take a look at the link below, it might be of some help.

Link

Water quantity on our job sites were for two purposes; one purpose was to control dust on dirt roads and the amount of water was based on street length, cloud cover, temperature, and relative humidity. The amount of water to be used was as needed but after a few days of hot weather you developed a sense how often you had to go to a hydrant to fill up; a second purpose was to control dust emission (primarily to limit the silica exposure) when cutting into concrete and again the amount was determined by the seat of your pants. The best advice I can offer is the experience that you develop on that job.