Not sure what you are asking. First, you need to know how fresh the fish you are purchasing is. Did it arrive in the store that day? From where? Does it already have a "fishy" odor? If so, take a pass. There is really no rule of thumb when it comes to fish. If it's REALLY fresh when you get it home, and you have removed it from its store wrappings and rewrapped in fresh packaging, it SHOULD keep for two or three days (MAX!) in the coldest part of your fridge. An additional precaution you can take is to fill a bowl with ice water and lay your package of fish on top (wrapped in plastic). Still, three days is max. I wouldn't chance it after that. Oh, and BTW, I've been teaching fish cookery for more than 30 years, so I hope you'll take my word for this. :)
My rule is day one..it can be sushi, day two..needs to be cooked, day three..I am suspicious, but I still may cook it.
Agree with the others. The only way I will keep fish longer is when it is caught fresh and kept iced in the refrigerator. Then 5 days is probably the most. (redfish and speckled trout).
You're getting good advice, but I did want to add that most fish bought from a fish market can't be sushi, even on day one.