Some guys let thier Ecallers play constantly, and some guys turn them on and shut them off randomly or when they see a coyote coming. Both ways seem to work.
One thing that I have noticed, although I have rarely been around the use of an Ecaller, but for those that turn them on and leave them on, it seems like the coyotes come faster/harder to the constant sound. Not always the case I'm sure, but it does seem to be a pattern.