You should serve and eat your shrimp sushi as soon as possible after preparing it. However, if you prefer your sushi stacks to be a bit colder, it is perfectly okay to transfer them to the refrigerator covered for about an hour before serving.  Just make sure that they are being consumed within 2 hours for the freshest and tastiest results. After that amount of time, the avocado will begin to brown and the rice will start to get mushy.  If you do end up with a leftover stack or two, don’t fret. You can save them safely in the refrigerator, in an airtight container, for 1-2 days.