I just figured I'd turn the light on in the morning (around 8 am) and then turn it off at night (anywhere from 7-9 pm).
The tank is a 30 gallon tank.
This is a riparium-style tank. There will not be any aquatic, fully-submersed plants - only plants like peace lilies (leaves above water, roots submersed) - so I don't think water depth should matter too much. The tank is going to be full, so the light will be suspended above the tank, something like this: