It's important to let your fish sit in the citrus juice for at least 10 to 20 minutes to properly cure. Anything under that amount of time and the fish will still appear raw. Not Getting The Right Ratio Of Acid To Fish. Flickr/Sterlic. You definitely need enough acid (lemon or lime juice) to properly make ceviche.dEeBb via Getty Images. Most chefs recommend marinating fish for ceviche for at least 10 to 20 minutes. It ensures that the fish properly absorbs the citrus juice and turns an opaque color. Anything over an hour, and you run the risk of the fish falling apart.