They would have been released as part of the Treaty of Brest-Litovsk on March 30th, 1918. In Article 7 of the Treaty:

The prisoners of war of both parties will be allowed to return home. The regulation of questions in connection with the above will be the subject of special treaties mentioned in Article 12.

Following that, referring to Article 12 it states that:

The reestablishment of public and private legal relations, the exchange of war and civil prisoners, the question of amnesty as well as the question regarding merchant ships which have been seized by one or the other side, will be provided for in separate treaties with Russia, which form an important part of the present peace treaty, and as far as it is possible come into force simultaneously with the latter.

Subsequently there was another treaty establishing Consular Relations between Germany and the Communist Government. Which outlined that the prisoners would be returned promptly, especially those unfit for military service. The ones fit for military service would be released at intervals established by both governments.

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Now, that what was agreed upon in March of 1918. However! The Treaty of Versailles kind of complicated the Treaty of Brest-Litovsk. Additionally some of the forces in POW camps were used by the Soviets in the Red Army as they desperately needed trained men. Now, this did not necessarily run against the Treaty of Brest-Litovsk since it stipulated that the persons could decided to stay in the country they were imprisoned in. Also, as stated, the Treaty of Versailles and the fall of the Second Reich definitely complicated things legally. What is interesting is that a large contingent of Hungarian Prisoner's of War joined the Red Army willingly. Either to escape retribution by White Forces, the Czech Legion, or to repay the Bolsheviks for releasing them, or maybe due to lack of other options.

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While they did help fight in the Civil War, they did mostly end up back home after the war in 1922.

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