[logfromblammo]

It depends on the chemical.

Protein-based drugs, such as oxytocin or insulin, are likely to be consumed and recycled into component amino acids.

Most molecules left out in the environment will eventually be oxidized, have an important bond cleaved by UV light with enough energy, or undergo thermal decomposition.

As sunlight, dissolved oxygen concentration, and temperature can vary wildly just by moving a few meters away, and the chemicals themselves have different stability, it is very difficult to predict how long a pill that was flushed down the toilet will persist in the environment.

Generally speaking, certain drugs are only useful because they take longer to degrade inside the body. It wouldn't do you much good to get an injection of a drug if your body's proteases chop it up to uselessness in the first ten minutes, or if it zooms right to the liver and gets methylated, or whatever it is the body does to clear foreign chemicals. In those cases, microbes will also have a hard time turning them into something else.