The field marshals and generals captured at Stalingrad were treated relatively well, with their own quarters near Moscow, but the rest of the army prisoners were marched to Prison camps on the Steppes and the Ural mountains near Siberia as well. The prisoners were often made to walk through the plains and snow by the Russians, and those that were too slow and weak were often shot. If they were unfortunate enough to pass by a hostile village, they were usually beaten and robbed by angry mobs on the way.
The German Sixth Army was eventually scattered to more than twenty camps from the Arctic Circle to the Southern Deserts. Some were marched, others were herded into trains. One train carried thousands of Germans from the Volga to Uzbekistan. They basically crammed the prisoners inside with little food or water, and they would often resort to killing each other for scraps of food. Another train that was destined for the Pamir mountains had almost half its passengers dead on arrival.
A few Germans remained in Stalingrad to reconstruct the city, but they were hardly cared for either. Typhus killed many and it was recorded that the Russians buried forty thousand corpses in a mass grave in Beketovka by March.
As you can imagine, having starving men crammed into these prison camps was a recipe for disaster. It was estimated that from the three month period of February to April 1943, over four hundred thousand prisoners (German, romanian, Hungarian, Italian) had died. The Russians simply let many of them starve to death. Camps would receive food trucks every third day, and by that time the inmates were beating each other to death to eat. There were instances of cannibalism amongst the soldiers. It became so bad that anti-cannibal squads of Captive officers were actually armed with crowbars to hunt them down.
Others were more creative with their survival methods. A group of italian soldiers who were locked in Ice Cold Rooms actually propped dead corpses up in chairs and pretended to engage in conversation with them. The guards made a daily count of the ‘prisoners’ in the cell, and the still-living prisoners ate well from the extra rations.
The treatment of prisoners started getting better by May 1943. Nurses and Doctors were sent to the camps to care for the survivors, and political agitators also sent in to indoctrinate the prisoners against Fascism and to become pro-communist. In most cases, those who turned against Hitler had a specific goal in mind. Cooperation meant extra food.
As for the period of internment, someone else with closer sources can confirm, but it seems like the prisoners were released gradually over the years, the first trickle of prisoners started being released after the Berlin Airlift in 1948. It was to such an extent that by 1955, there were only 9,626 prisoners left in the camps that were directly connected to Stalingrad (with 2,000 having actually fought there)
For these prisoners, it was West German Chancellor Konrad Adenauer who got the process started. He flew to Moscow to meet with the leaders of the Soviet Union in 1955 to plead with them to release the prisoners.
Moscow’s stance throughout this period was that they no longer held German Prisoners of War in the Soviet Union, only war criminals of Hitler’s armies, ‘convicted’ by Soviet Courts for crimes against the Russian People in General. But after negotiations with Bulganin and Khrushchev, Adenauer was eventually able to secure the release of the last of the Stalingrad prisoners by September 18th 1955, who began their final journey home.
[Source: Enemy at the Gates : William Craig]