The time required for par cooking potatoes really depends on a variety of factors. How big are the potatoes? Are the potatoes cut or diced? What temperature are you planning on grilling the potatoes at? Is your grill properly calibrated to the correct temperature? Are you going to be cooking anything else on the grill at the same time as cooking the potatoes? A general rule of thumb is to parboil potatoes until they are fork-tender but not falling apart, this can range anywhere from 15 minutes for small potatoes to 25-30 minutes for large whole potatoes. The best way to tell if the potatoes are done is to either taste a small portion or use the tip of a paring knife to check for resistance, if the knife goes in easy the potato is most likely cooked. For baked potatoes, the best method for testing doneness is an internal thermometer. 205 °F to 215 °F is desired for a fluffy baked potato. Par cooking methods can range from boiling to steaming, to microwaving, to pressure cooking. All these methods will vary in the speed and efficiency in which they will cook potatoes.