Assuming it’s being kept in an ideal environment, what edible (to humans) food item would still be safe to consume after the longest period of time had elapsed? And how long would that be? 50 years? 100 years? More? I really have no idea.
Tweenkies. They probably last longer than paper.
Hardtack remains edible for years if kept away from bugs and moisture. Here’s a picture of a piece that’s about 160 years old.
I’m guessing honey. It’s been found in Egyptian tombs, apparently still edible after a couple of millennia. I’d find the stories, but it’s a PITA from my phone…
Twinkies, of course! They’ll be what is found tens of millenia from now when future archeologists are excavating our civilization.
Seriously, though…
There have been cases of wooly mammoth and mastodon meat being so well preserved in glaciers that it was edible when the animal emerged from the base of the glacier. Most instances of these events state that the meat was rotten when found, but dogs and scavengers would still eat it. There are persistent reports, however of native americans eating such meat, and reports into the 20th century have explorers finding and eating from such carcasses.
See, for instance, this.
Also, seeds and grains can be stored for a very long time if they are kept in a cool, dark, and very dry place and packaged so that oxygen can’t circulate around them. I have no idea how long “long time” is in this context but I believe it to be measured in at least decades.
Specific foods, such as pure honey, will keep indefinitely if properly stored.
…there’s also a story about how people have found honey in ancient Egyptian tombs that was thousands of years old, and still edible. I can’t find any reliable cites for this though, and suspect that it may be a myth. It got me thinking though. Any pure sugar (table sugar, dextrose, etc) should last indefinitely if kept in a dark, dry, temperature stable container. If you vacuum-sealed it in a gold-lined titanium container my guess is your descendants 100,000 years from now wouldn’t even notice the difference.
Of course, people occasionally drink wines that are 150 or even 200 years old, and the oldest bottle I’ve heard of being consumed was about 300 YO.
I’d think common salt would keep indefinitely assuming you could keep the elements and critters away. You might not consider it a food stuff, though it’s certainly edible and used a lot (too much).
As others have alluded to, it depends entirely on the conditions on your shelf. What will keep well on a shelf in the middle of Antarctica won’t keep well on shelf in a cave in the Sahara, and neither will keep well on a shelf in the Amazon.
There have been seeds found in mud on lake bottoms and in the permafrost that still contained living cells, that’s how fresh they were, they weren’t even dead yet. Those seeds were thousands of years old. But6they wouldn’t have lasted that long on a shelf in the Amazon. Neither would the thousand year old mammoth steaks have survived in the Sahara.
The question is utterly unanswerable unless you can be specific about the conditions of your shelf.
Almost any food will keep indefinitely if you can keep the elements and critters away. Salt isn’t special in that regard.
I was just reading about this in a recent “Time Magazine” article talking about food that lasts forever - the mythical Twinkies only last about four weeks. You’d do much better with canned Spam, which although the label says it expires in two or three years, will actually last 12 to 15 years. The food scientists at NASA are doing a lot of very interesting research in this area.
The article in question (for Time subscribers only, unfortunately).
For the non-subscribers, what does it say about honey? It’s shelf stable in any climate, even if it crystallizes or freezes, or stays warm. Honey has my vote.
:nitpick: I wouldn’t consider salt to be a ‘foodstuff’. It is something that the human body requires, but alone wouldn’t sustain life for very long. :nitpick:
Honey gets my vote, also. I don’t know how long a human could survive on honey alone, but I would hazard a guess that it would be considerably longer than a diet of just salt. 
I think there are some Doper nutritionists, perhaps one of them may have more to offer on the subject of how long one could survive on a diet consisting solely of honey.
I suppose it depends upon how fussy you are regarding food. Strain 2-9-3, or B. permians is a bacteria discovered in an ancient sea-floor at Carlsbad, New Mexico. The bacteria is 250 million years old…and it lives. Yummy. 
Yeah, but is it edible? :dubious:
Depends what he meant by “critters”. I took it to mean anything from insects up (i.e. not microbes).
I doubt he was saying that salt, if perfectly preserved, lasts indefinitely because that constraint is not necessary.
I would think dried pasta, kept in a sealed container with some desiccant, ought to last for a very long time. Like hardtack, it’s mostly made of flour and water, with the water ultimately being removed before storage. Living things need water, so keeping a food item as dry as possible greatly aids preserving it for the long haul.
I was - first line of the OP: “Assuming it’s being kept in an ideal environment…” (whatever the ideal environment for that particular thing would be).
Kimchi
Honey
We have someone at work that has two Twinkies sitting on the top of her bookshelf, wrapped in clear cellophane. They’ve been there about six months. No visible signs of changes or deterioration.
Dried rice stored in plastic… probably last 10 years or more.