How long does psychosis generally last?
Depends what has caused it...
Psychosis is a very broad definition, and can range from slight abberrant occasions to life-lasting mental disorders (e.g. Schizophrenia, Bipolar Disorder)).
If it is due to brain injury : life time
If it is , say, Drug-induced : can be temporary, lasts until the substance has cleared the Neural receptors it is destabilising..
Note : some drugs (e.g. Alcohol, MDMA etc) can have long-term (read: lifetime) effect by causing permanent brain damage..