If it's the only light they get, it doesn't matter when you turn it on. If they also get sunlight/daylight, then I'd turn your light on about mid-day.

If your snakes are already eating, make sure the temperature is warm enough so that they can digest their food. I don't know if you're using separate lights to provide light and heat, or if you're using an under tank heater. If it's not warm enough, the food could start to accumulate in their stomachs and lead to a bacterial infection, regurgitation, and illness.
Rick

P.S. Congratulations on getting them to eat. I bet you're relieved.