Answer: 30 days.
Step 1: When a Magistrate is satisfied that an alleged mentally ill person needs to be detained in a psychiatric hospital, he passes a Reception Order for admission and detention.
Step 2: This Reception Order is valid for 30 days. If the Magistrate is not satisfied, he may refuse the application, giving reasons in writing.
Step 3: Do not confuse this with admission under special circumstances, where a mentally ill person who cannot express willingness can be admitted for 90 days on an application by relatives or friends. The Magistrate's Reception Order limit, however, is 30 days.
Ref: Mental health law, Reception Order provisions.