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

[Metroyeti]

If you live in an urban chav enviroment, the normal folk will try and flee when tshf, chavy scrap men tear house to bits for all the scrap, dont know what there doing and cause a gas leak. Ig you do this over a large area there is a risk of these going kaboom. Thats not to mention the random bits of arson that already goes on. Then the looting will start with more arson, we have had riots in the north east before,and im proud to say we didnt join in with las years london riots.