With schizophrenia the pattern is usually one of treating a first psychotic episode with medication for a year or two and then reassessing the need for medication. Many things can cause a psychotic episode, including drugs, bipolar diosrder and even depression. People diagnosed with schizophrenia are often on a maintenance dose of medication to prevent a relapse of psychotic symptoms for the rest of their life. I'm one of those people. A first episode usually responds very well to a very low dose of medication but years later the dose of medication needed is considerably greater and the response not so good.
Nobody is quite sure why, but many researchers believe it is an effect of repeated relapses.
A response to medication can be apparent the next day in some people. The standard hospitalization in Ontario is about two weeks. The drug trials to determine response curves to new medications typically go for six to eight weeks. With some medications like Clozapine it can take six months to get a response and with the atypicals you generally see continuing improvement for two years or more. They don't do many studies longer than that.