Ok man, if you have to ask it's been too long, don't eat it
Go to Red Lobster
It's impossible to answer this with a time, because there are too many variables. What's the temperature in your fridge? What kind of fish, warm- or cold- water? White meat, or oily? How long was it out of the water before being iced? How was it transported, and at what temperature? How long was it on display in the store before you bought it? Really fresh fish, properly wrapped, might keep 1 to 2 days.
Rather than asking how long it might last, it's better to ask how to tell it it's too old. Good fish will be firm to the touch, not soft. The flesh will rebound, not dent with a fingertip. Any scales will be shiny and tight, not dull and loose. Gills will be pink or red, not brown or black. Eyes will be clear and shiny and bulging, not cloudy and shrunken. And any smell should be mild, not strongly fishy. If you have just a skinless fillet, use the smell and texture...if it fails any of these, throw it out! Things like E. coli, listeriosis, or yersiniosis aren't worth the risk!
do you mean before it smells up the entire fridge and everything in it- this will happen even before it spoils. I would suggest not to save left over fish in the fridge.
maybe a couple of days. If it smells strongly fishy, thats not a good thing. Fish is gross must faster than beef and chicken and pork. Pork lasts the longest