Diablo III: Reaper of Souls

How often should I be recasting Blizzard? What's the chain length for the DOT?

I generally recast Blizzard every few seconds when moving through trash or fighting the Rift Guardian. The DoT leash is ~2 screens away for everything, but the rift guardian, which leashes about a screen length when you're solo. When in a group he probably follows the same "2 screen length".