It depends on the type and thickness of fish you are grilling, but generally speaking, most fish will be cooked through in about 10 minutes. Keep in mind that you'll want to preheat your grill for at least 15 minutes before cooking, and that thinner cuts of fish will cook more quickly than thicker ones. When in doubt, use a meat thermometer to check for doneness - the internal temperature should be between 145-160 degrees Fahrenheit.