It depends on how thick the layers of fresh and salty water are, and whether there's anything mixing them together. The molecular diffusivity of salt in water is about 10^-9 m^(2)/s, which means that the distance salt can diffuse is roughly sqrt(10^(-9)t), where t is the time in seconds. So, for instance, if you had a 100m thick layer of freshwater lying on top of a 100m thick layer of saltwater, it would take well over 100,000 years for molecular diffusion to fully mix them.

In practice, in the ocean, there is mechanical forcing that causes mixing (e.g. the winds, tides, turbulence, etc). A typical value for the real, effective vertical diffusivity in the ocean (taking into account mechanical mixing) is 10^-5 m^(2)/s but, even then, it would take several decades to mix these layers together. And 100m is pretty small compared to the thickness of deep-ocean water masses.