I've personally found that it's better to constantly scan when night hunting. The main reason is because it only takes a few seconds for a predator to go from "out of sight" to "in your lap". This seems to always happen while the daggone light is off!
With that said though, I scan a couple times around my area, then cut off the light for about 30 seconds to a minute in most cases. Most rechargable spot lights don't have very much battery life, so they won't hold up to very many 15-30 minute stands if you burn them constantly.
The worst feeling in the world is to see eyes coming in, only to have the light die before you get a shot. I carry 2 or 3 lights in the truck. I'll use one, and keep the others charging between stands.