How long before fresh scallops and fish go bad?

How long can you keep fresh scallops and or fresh fish in the fridge before it goes bad? As a rule of thumb...I know what they look like when they go bad and what they smell like... I usually buy it the day of or the day before I make it..I'm just curious to know how long I can keep it in the fridge before its no good..in case say I buy it and the the next night I'm going to make and I get a call to go out to dinner with family..etc.. Thx