How well do Swedish fish keep past their best by date?

They will stop looking edible before they stop being edible.

It probably won't make you ill immediately, more likely the texture or flavor would begin to suffer first (hence "best by" rather than "expiration" date). Keeping it stored properly (i.e. not an open bag but something sealed) would likely allow it to last longer.

You should probably not eat 3.5lb of candy within 10 days unless you are trying to make your intestines suffer, but if you choose to binge please update us as to the state of your health so that you may be used as a cautionary tale.

"Best Before" is not an expiration date, it's just so that if you aren't satisfied with the quality after that date the company can say, hey we warned you, we can't guarantee they'll be up to our standards that long.

They aren't going to suddenly go rancid on November 14

Since you already received the genuine answers:

You need to be really careful. The expiration date isn't exact, but after that, they'll quickly ferment and turn into Surströmming on the inside.

"best by" isn't an expiration date, it's just a product suggestion.
Only a very few things, like baby food, have an actual expiration date that's meaningful.

Treat them like any other food. If it looks fine, smells fine, feels fine and tastes fine, it's probably fine.
They're basically entirely sugar, so the biggest worry is going to be drying out, not spoiling.

In the US, the "best by" date means nothing. They just want you to throw it away so you buy more. If it looks fine, smells fine, and tastes fine, it's fine.

Probably for awhile, it's a hard gummy candy. They'll harden but I'm not even sure they can go bad.

Swede here. As a certified specialist in Swedish fish storage I recommend the following:

Divide the 3.5 pounds into smaller portions and store in separate containers (fx Ziploc bags).

That way they won't go stale as fast since the unopened bags will retain the moisture.

Best before dates codes are more what you'd call guidelines than actual rules. Yar

Quit well, I imagine.

worst case scenario, they get hard?

This got me so confused, and I'm Swedish lol.

You should probably just eat them all. Right now. In bed.

It would take an awful lot of time or other circumstances for swedish fish to become inedible. They'll just get "stale" (with gummy candy, this is usually just getting a bit harder/chewier) and, in my experience with gummy Lifesavers, the flavor becomes more concentrated. I actually like that stuff just a little stale.

You'll be fine. That's not an explicit expiration date. It's just a date to be guaranteed a minimum quality.,

They'll probably be fine, might just get a bit hard.

Those things have so many preservatives in them that they will probably be discolored, have a bad texture / taste, or be rock hard long before eating them makes you physically sick.

I'll just leave this here.

Swedish Fish are garbage, dear friend. They are like a depressed dystopian robot's impression of candy. If we're going to kill ourselves eating overprocessed sugar, it should at least be tasty. Even the simple step to Gummy Bears marks a vast improvement.

A vacuum sealer is your friend. I buy giant bags of everything now and split it up into single serving portions that last a long time.

You'll be fine (like everyone has said).

Honestly there are some candies that I prefer stale (usually past the best by date), chiefly Gummi Baren and Tootsie Rolls. Just somehow more satisfying once they're less soft.

Not familiar with Swedish Fish specifically, but generally candy is mostly sugar, which is a preservative in itself so it doesn't really go off. Also not familiar with the regulations in your area, but in my country foods are generally good for at least three months outside of a "best before" date if kept in their original packaging or at least airtight. Flavour and texture might decline a bit, but they won't make you sick. It's the "use by" dates you want to be observant of.