What is the longest time required by law for a case sent back by a higher people's court to an intermediate people's court for retrial?

Six months.

"People's Republic of China (PRC) Civil Procedure Law" has corresponding provisions:

Article 149? A case tried by the people's court through ordinary procedures shall be concluded within six months from the date of filing the case. If there are special circumstances that need to be extended, it can be extended for six months with the approval of the president of our hospital; If an extension is needed, it shall be reported to the people's court at a higher level for approval.

The people's court shall pronounce a judgment in public on cases that are tried in public or not. If a verdict is pronounced in court, it shall be served within ten days; If the sentence is pronounced regularly, a written judgment will be issued immediately after the sentence is pronounced.

? Extended data:

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Relevant provisions of the Civil Procedure Law of People's Republic of China (PRC):

Article 145 Before the verdict is pronounced, if the plaintiff applies to withdraw the lawsuit, whether to grant it or not shall be decided by the people's court. If the people's court decides that the withdrawal of the lawsuit is not allowed, the plaintiff may make a judgment by default if he refuses to appear in court after being summoned by summons without justifiable reasons.

Article 146 The trial may be postponed under any of the following circumstances:

(a) the parties who must appear in court and other participants in the proceedings have justified reasons for not appearing in court;

(2) The party concerned temporarily applies for withdrawal;

(3) It is necessary to notify new witnesses to appear in court, obtain new evidence, re-authenticate, conduct an inquest or make supplementary investigations;

(4) Other circumstances that should be postponed.

Baidu Encyclopedia-People's Republic of China (PRC) Civil Procedure Law