I hunt gray fox in Maryland and use the Johnny Stewart caller. Usually, I' ll start with the volume about 50% and if nothing comes running within 5 minutes, I' ll crank it up and reduce the volume every several minutes until something comes in or I' ve reached 15 minutes- whichever comes first. Setting up away from the sound source, sitting still, using a lens filter, and shining quickly and sparingly are essential.
What type of predators are you calling? As I said, I hunt gray fox, but there are no guarantees about how long it takes before they come in. I' ve found that usually if they' re coming in, they' ll arrive within the first 10 minutes... They also are just like people (some are pretty sharp while others have the IQ of a donut hole!!!)

Jim