I start callin' as soon as I sit down. No reason to wait. Some say to "let the woods settle down", whatever that means. IF you got in your spot without the coyotes knowing you're there, then sit and call. If the coyotes know you're there, they aren't coming anyways.
Call off and on for at least 15 minutes. If you don't have lots of places to hunt, then by all means, sitting longer won't do any worse than kill some time.
Once you've called off and on for at least 15 minutes, you might sit another 5 quietly, something might slip in. Most of us have lots of places to call, so it becomes a numbers game. The more stands that can be made in a day equals better chances of calling something in by moving on as soon as you're done calling each stand.

Biggest thing is to make certain that you have the wind in your favor. Make absolutely sure that the wind isn't in a direction that a coyote can come and smell you and leave without being seen.