Egg Foo Young will last in the refrigerator for up to 4 days before it spoils and should be thrown out.
If you have left the Egg Foo Young in the refrigerator longer than 4 days then you should throw it out because bacteria can form rapidly on the Egg Foo Young even though it has been refrigerated.
If you want to keep leftover Egg Foo Young longer than 4 days you should freeze it instead.
When frozen the Egg Foo Young will last for up to 6 months.
Then you can just thaw it out in the refrigerator or in the microwave and eat it when ready to eat it.
Leftovers should not be left longer than 4 days in the refrigerator because they can go bad and grow bacteria.
If you have leftovers that you don't plan to use within 4 days you should freeze them for long term storage.