what's the most past a food items Best Before date that you will eat something?

I think the general benefit of abiding by the suggested "best by date" is avoiding a heightened level of bacterial contamination. All foods have some  level of bacteria, the longer it sits the more time that bacteria has to cultivate. After a Package or container has been opened, there are even more bacteria culture introduced to that enviroment thus increasing the level of bacteria that makes up the biosphere of life on the surface of that food product.. . Which ultimately could potentially cause the food to be overcome with bacteria .. Otherwise known as it "going bad". That is when it can become a potential health risk and perhaps even make someone (who had consumed it) sick.

Any canned food. I've eaten stuff that was 2 years past the use by date. As long as the can isn't puffed out, it's probably good.

Most items can be safely eaten 7 days past their due date. Especially if they're properly refrigerated.

The "best by" date amuses me somewhat---it's like the packager of the food doesn't know what it may grow up to be after that date and doesn't want to take responsibility for its actions after that----but there is obviously some component of judgment with regard to how long before it turns "harmful" for human consumption.

For me, it depends on the food---within "reason."  I've heard that they tested some of the supplies found in old fallout shelters from the 1960's and found them to be still quite edible.

Soft food items---bread, cheese, etc.---are more problematic.  Mold is a real potential problem, and cutting off the mold that you see doesn't always get it all.

Sometimes the odor changes sufficiently to suggest it best be thrown away also.

And in other cases, the temperature at which any food has been stored can be a factor.

"When in doubt, toss it out."

And, unfortunately, third world countries sometimes do not have the luxury of implementing such protocols for throwing food away.