Shrimp ceviche marinating time can range from 90 minutes to 2 hours for raw ½-inch chopped shrimp.  Fish ceviche marinating time can range anywhere from 20 minutes to 1 hour for ½-inch cubed raw fish.  20 to 25 minutes will be medium and 25-35 minutes will be medium-well.  Anything longer will be fully cooked and bordering on tough.  When using raw sashimi-grade fish, it is better to error on the side of undercooking the fish so it stays more tender – just like sushi!