If it's salted, some experts say up to two weeks.
Americans refrigerate a lot of things that other countries don't, like eggs and butter. Eggs you may have encountered before in your travels—in most European countries, eggs aren't refrigerated. But butter? Most Americans keep their butter in the refrigerator. Heck, there's even a compartment in many refrigerators explicitly for that purpose. If you're like me, you might even get nervous if the butter gets left out on the counter. It's dairy, right? Shouldn't that be chilled to keep out all the bacterial grossness? But it turns out that keeping butter in the fridge is a matter of preference, not of food safety.
There are some caveats, but, according to some experts, salted butter in an airtight container, like a crock, is perfectly fine to leave out on the counter for up to two weeks. That's right: two weeks that you can enjoy perfectly spreadable butter for your crumpets and muffins and toast and whathave you, without worrying about the dread block of chilled butter interrupting your spreading game. (Though there are some easy hacks for spreading cold butter too.) The USDA and FDA don't take a firm stance on how long butter last, and other experts recommend only leaving butter out for one or two days maximum. Regardless, it definitely means that you shouldn't be afraid to leave butter out for a few hours for optimal softening.
But for the butter to be safe for that long, it should be salted, pasteurized butter. It also won't work during the summer. If your kitchen is above 70 degrees, you should keep your butter in the fridge, no matter what kind it is. And if the butter goes rancid, then it's time to throw it out. How can you tell if butter is bad? Smell it. Like spoiled milk, rancid butter gives off a sour, unappetizing aroma, and may also have discoloration in spots. You won't want to eat it, so don't. But otherwise? Experiment with leaving out your butter, why not?