Light mover question

[Rickyrooz]

I am in currently in the planning stages of a frag tank system. I was thinking of using two 50 gallon Rubbermaid stock tanks and putting them next to each other. I wanted to use one 250 watt metal halide in a Lumen Bright Large reflector for my lighting. In order to cover the two tanks I planned to use a the Light Rail 3.5 6RPM motor. How long should my light cycle be? The light can travel the 6 foot distance in 6-7 minutes.