Both. Neither.
It depends on so many other things. One thing that you should be aware of, is the the purpose of acclimation is to reduce stress on the critter. Long acclimations can increase the stress of confinement and oxygen deprevation. Short acclimations increase the stress of temperature, salinity, pH changes. If the water in your tank is the same temperature, salinity and pH of the water that the fish is used to, it's better to cut acclimation short. If he's changing any of these variables, you should lengthen the time of acclimation.
My rule of thumb is to test the water that the fish is coming from and vary the time of acclimation based on differences between the water he's used to and the water he's headed for.