It varies a lot. "nervous breakdown" is an imprecise slang term. Average stay in a psychiatric unit is about one week for adults, but this varies from 1 day to many weeks. The average is longer, about ten days, for geriatric (elderly) patients, mainly due to co-existing medical issues. Patients may go to intensive outpatient treatment, eg, 3 or more days/week, after hospital & before regular office followup.
Nervous breakdown. Is not a diagnosis, so it's hard to say. In general, an admission to a psychiatric hospital only lasts 5-10 days. In a lot of instances, this is not long enough for the person to actually feel much better.