You can do it both ways, either intermittent or on for a full 30min duration (see repeat loop for electronics)
Try it and see what works best for you in your area...I happen to like the intermittent and will call for as little as 20sec every three to five min. Sometimes I am on stand for as little as 20min but you can get some of the older, wary dogs in by staying in place up to 40min.
Once I know that something is there, I like to shutup completley cept for some light coaxing and let them hunt me down.