How long post SHTF before a city is safe to re enter?

[Ghost]

Wanna know what happens to an abandoned city?? Check out Prypiat, the city abandoned after the Chernobyl disaster

http://en.wikipedia.org/wiki/Prypiat,_Ukraine

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Many of the building interiors in Pripyat have been vandalized and ransacked over the years. Because the buildings have not been maintained since 1986, the roofs leak, and in the springtime the rooms are flooded with water. Trees can be seen growing on roofs and even inside the buildings. All this adds to the deterioration process; a four-story school collapsed in July 2005.