How long will a drowned victim float before sinking?

They do not float until later. When a person drowns, water takes the place of air in the lungs. The body becomes heavier and sinks.

It stays there until enough gas builds up inside the body from bacterial decomposition to make it lighter and free it from the suction that silt and mud creates on the river bed (if that is where it happens), so that it surfaces.

The time it takes depends on a number of factors: air temperature, depth at which it came from. The amount of sunlight can also affect the length of time at or near the bottom. If a water is very cold, it may never rise.