How long does it take for milk to spoil unrefrigerated

Buttermilk is already thoroughly packed with live bacteria. During its manufacture, that bacteria already consumed some portion of the available lactose and turned it into lactic acid.

Because of the lack of food, acidity, and the extreme competition it is pretty hard for buttermilk to go bad. The good bacteria will stay active and the buttermilk will get thicker and more sour until it runs out of lactose. In fact- when your buttermilk container has about 1/2 cup left you can make more just by refilling the container with milk and leaving it to ferment on the counter for a day. If you use it up more quickly than the bacteria eat the lactose then you can keep this up indefinitely.

Don't worry if it is thicker- if it still smells good then it probably is. As Noctrine said- mold around the lip is the worst risk. I am not a food chemist and despite my personal experience- if you ever suspect that food is bad just throw it out. $2 of buttermilk isn't worth an unpleasant afternoon.

The term "bad", when applied to cheese, can actually refer to a great many defects (p. 14), several of which are caused by improper or over-aging and only some of which are dangerous. The issues that you most likely need to worry about are:

1. Moisture loss applies primarily to soft cheese and is probably the first negative effect you'll see. In a sealed package, it will "sweat" as Ivo points out, caused by water evaporating and then condensing again on the packaging, forming little droplets. Unsealed, it will simply dry out and become hard.

   This is not spoilage, and the cheese is still perfectly good to eat, it just might not have as pleasant a texture and might have a slightly flat flavour. You can just cut off any hardened edges.

2. Oxidation gives a slightly metallic taste but more importantly leads to rancidity - discoloration and a sour smell/taste. This is caused by light and is why you should store cheese in a dark place. If you left it in the car during the afternoon, it may have been exposed to a good deal of light. However, proper packaging greatly slows oxidation and wax paper in particular is an oxidation inhibitor (that's why cheese is often wrapped in it!). Light exposure also would have been minimal at night. You're probably fine in this respect, but use your nose. If it smells sour, don't eat it.

   The chemical process itself results in peroxides and free radicals, which are obviously not good to eat, although there seems to be some debate about just how dangerous they are (some say it raises the risk of cancer, although I'm unable to find an authoritative source, and this might just be an urban myth).

3. Mold is the most obvious form of spoilage and usually also the last. Although most mold is technically aerobic, many strains are capable of growing with very little oxygen, and I personally have seen mold growing on an unopened package of cheddar that I'd forgotten about for several months. I'm sure I don't need to tell you that mold isn't safe to eat; however, by the time there's enough mold to be dangerous, you'll be able to see it.

   Contrary to what Ivo's post suggests, the mold you'll see growing on spoiled cheese is not the same as the mold that the cheese was made with - some cheeses, like parmesan, aren't made with any mold at all, although they can certainly grow mold later on. The molds used in cheese-making are typically some variety of penicillium, which is safe to eat. The mold that grows weeks later could be anything, and is not safe to eat. Ivo is also partially correct in saying that the packaging does not prevent mold growth; however, it does slow it down by restricting the available oxygen.

On the whole, I tend to agree with the other answers that your cheese will almost certainly still be good after only 1 day. Keep in mind that cheese is thought to have been popularized as a method for preserving milk before refrigeration existed - the whole idea is that it lasts a long while, usually well over 24 hours before food safety becomes an issue.