I use a baby boom-box (sometimes) with a tape. I have about 100' of speaker wire, and thus can set the speaker out front and control the length of time of the call and its loudness from where I'm sitting.
The majority of my experience in southwest Texas is that if Ol' Wiley doesn't show up fairly quickly, he ain't gonna be there at all. I guess I'm talking a minute at most of tape-time, and three or so minutes of waiting and possibly three repetitions of this.
With a hand-held call, I yowl until I'm sorta fed up, maybe 30 seconds to maybe a minute at most, then wait. There's no hard and fast rule about this. If you see a coyote, reduce the volume. He might run off if he thinks that rabbit weighs 400 pounds.
Coyotes tend to come in to maybe 75, maybe 50 yards and then veer downwind to smell the goody before they move in. Take this into account when you pick a sitting-spot, so you can see clearly into the downwind area. That's the why of having a speaker out in front of you, cross-wind.
Art