What is the appropriate time to wait before going cannibal in a shit hits the fan scenario? You want to make sure that it's not just a local blackout or something, but you also want to be able to stockpile the best looking cuts of meat (and preserve them for jerky and such) before they get too lean. . .
Well, I think there's really two different questions here.
1. At what point is it OK to eat human meat I just find/buy?
There comes a point in any major apocalypse where the availability of freshly dead humans from non-human-hunting incidents starts to become the preferred alternative to yet another night of boiled boots. I think that point is once I've given up on getting laid. For those paying attention, the only thing keeping me from eating human right now is availability and what prospective sexual partners will think of me.
2. At what point is it OK to start hunting humans for meat?
As Captain Reynolds put it, "If somebody tries to kill you, you try to kill them right back." And then, you know, if you're hungry. That's just good hunting etiquette (you eat what you kill).