When did the food reserves run out in Leningrad? Also how common was cannibalism in the city, and if one resorted to it how likely was the person to get caught?

The buttocks of a few corpses were reported as being 'harvested' but really it was just rumour, the most extreme example of what people did to stay alive are two of my favourite "darker than the darkest thing ever" history facts.

The local river and lake were a source of fresh water, unfortunately to the residents they had been the site of a massive fight and were littered with dead bodies which tainted the water before they froze over stopping the process. The net result were hungry russians attempting to melt what ice they could cut into water that was sweet with decay, that or they ate the ice and died of pneumonia.

A supply warehouse was shelled early in the siege setting fire to the sugar supplies severely hampering rationing efforts for the Soviets. Towards the end of the siege people discovered they could eat the soil underneath the former warehouse as the sugar had melted down and collected as a substrate. Harvested soil could then be fried in paint thinner to melt out the sugar or just eaten raw.

Not quite cannibalism but I believe the efforts people went to for food shows that they did resort to cannibalism lightly if they ever did.

Reagan, Geoffrey. Military Anecdotes (1992) p. 12 Guiness Publishing ISBN 0-85112-519-0