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

[Skvez]

I think it's going to depend very much on the nature of the event and the thing you are going back into the city for.

If there was a pandemic and you managed to isolate yourself from it (rather than being infected and recovering) then re-entering a city is a really bad idea.

If it was a financial collapse and much of the population are still alive on barely adequate government rations then again entering the city might get you in trouble with either gangs or the local militia but there might be some economy (even a black one) for you to trade for whatever item you want (probably a tool as most other items will have been consumed). The risk there is probably being mugged, detained or followed back to your home.

An interesting aspect of one of the resident evil movies was where the roving band of survivors had picked clean all the small villages and towns and had no alternative left but to go into a big city. I can see that scenario happening in certain cases where people don't set up a retreat with adequate food production.

I don't see it being possible for us to come up with a date. It's going to depend on:
Nature of the event
Robustness of Survivors in the city (if we even know!)
Disease outbreaks (if we even know)
Urgency of the item.