Any time of day is a good time to call. As for length, ive never had anything respond to a call after 30 minutes or so, but you never know. People say most coyotes come in 5 minutes, and 10-15 is typically the maximum amount of time people spend calling coyotes. Well today i was cold calling for bear outside of saint maries and at 19 minutes i heard a noise behind me and turned my head and there was a coyote not 3 yards from me. Man he turned inside out fast, didnt even have time to swing my rifle his way.