Every stand can change. I will spend anywhere from 15min to 30min. Coyote's can come from quite a distance and they can also be in your location right when you start calling.
Always start your calling softly so you don't startle the close coyotes and scare them off. I have had a lot of coyotes come in within 2 to 5 minutes and they are really coming in fast. Sometime when I get up to leave I see wet spots in the ground and paw prints that weren't there before. I was taught to understand that the coyotes are always there. You are in there house and it is up to you to coax them in for a quick meal.
I will hunt a stand for 15 to 30 min. but it depends on the area I am hunting. Some of my "SECRET SPOTS" (wheels keep those a secret HA! HA!) need a solid 30 min. I think because of the coyote territory thing that goes on between packs. I have had them hang up and bark at me in plain view and not see me but refuse to come in. But if you are there long enough they get the courage to run in.......the whole time urinating on every bush and looking out for the other yote's from the other pack on the way in to the call.
Usually 20 to 30 minutes at one spot if nothing comes in. If something comes in and I get a shot, I'll do a quick, loud ki-yi for 30 seconds or so and then go back to a jackrabbit or cottontail call for an additional 15 minutes.