Depends on how much time you have. If you can call for 30 minutes at every set and still have enough time to cover the area you are hunting, then call 30 minutes or longer at each set.

If you don't have enough time to make all your sets calling that long, then shorten it down some. Unless I were entered in a hunt, I would never spend less than 15 minutes at each set. That gives most coyotes plenty of time to show, but you will miss a few that will show up after you leave. I have shot one coyote an hour and fifteen minutes after I started calling. You are just playing the odds.

If you are hunting the area regularly, spend more time at each set, and space out your sets over a period of days or weeks.

Most of the time I'm calling, I wait about 5 minutes after I get to a set to let everything settle. Then I will spend at least 30 minutes calling. Most coyotes show up in the fist 10 minutes of calling, but I've got a lot of bobcats in the neighborhood, and they are usually very slow getting there. Coyotes will run to a call. Bobcats usually creep from bush to bush, stopping frequently to take long looks around. I still have plenty of time to hunt the area in a weekend, and I won't have missed many animals showing up after I leave.