How long does it take to treat mental illnesses?

It is always an individual matter. For example, in schizophrenia and related psychoses it is a general principle to administer a medication for a year or two after the abatement of the first psychotic episode. After the second episode this period is five years long, and after the subsequent exacerbations of psychosis, treatment should be long-lasting, or even of unlimited duration.
Bipolar disorder requires an intensive treatment of antidepressing or antimaniacal nature depending on the phase, whereas between the exacerbations a treatment stabilizing the patient’s mood. After the expiry of their pathogens, reactive psychoses do not generally require maintenance therapy after the abatement of symptoms.