A lot of cheeses are naturally brined (feta, for example), and marinating cheese is not much different. For the best effect:
- Pick a porous cheese
- Cut off the edges if the cheese has a skin
- Cut into smaller pieces to increase the penetration
- Press it dry with towels (or paper towels)
- You can inject the cheese to get more flavour in it
- Marinate in flavours that compliment the cheese
- Let the cheese soak for at least 4 hours (more for less porous cheeses)
If the cheese is really wet (like feta), add weights to the drying process (like a pot or heavy plate), and let it drain until the surface feels dry.
Note that many cheeses are already salty, so balance your marinade with this in mind.
Food quality isn't binary; it doesn't go from perfectly good to perfectly bad in an instant. Even if it did, the time it would take depends on the storage temperature. And for non-liquid foods, it's possible that only a part went bad (how well does it mix?).
So, you don't get a precise date, but a rough interval at which time the decay starts to set in. As a result, the manufacturer will just pick a rounded date from that interval.
Example: the engineers might calculate that under reasonable circumstances, the product may start to noticably deteriorate after 52-75 days, and become dangerous after 81-112 days. They manufacturer could then say that the expiry date would be 60 days.
(The other answers explain why you'd use the first interval, but not why they're actually intervals.)
:edit:
The likely reason why they're all the same 60 days is probably also engineering. How much preservative do they use? As noted in the comments, the primary preservatives are the acids, but you need quite some sugar to compensate. It seems 60 days is a commonly accepted balance.
Best Answer
Cheese is not made from "rotten" milk, let me clarify that. Rotting is an uncontrolled process in which bacteria, molds and other life forms colonize milk, eat it, release waste into it and die. The resulting, rather unpredictable, crud we call rotten (or more precisely spoiled) milk.
Most cheese is the product of highly controlled action by bacteria that produce acids that coagulate the casein in the milk. The type of bacteria, the temperature, the amount of time they are allowed to act, the amount of water you drain out of the curdling milk, all control the end result in terms of texture, taste and flavor.
The kicker, though, is that cheese is by no means a sterile product - not even cheese made with pasteurized milk. Bacteria remain inside the cheese, and of course bacteria (and molds and yeasts) land on its surface through its processing and shelf time. While the action of these bacteria can be slowed down by cold and dryness, most cheeses will go bad after a while. How long is "a while"? It depends. At room temperature, mozzarella will go bad in a matter of hours while an aged unopened Parmesan may sit happily on a shelf for months and even years.
(apologies for the short and brutal definition of cheese: I have omitted curdling by other methods and the various surface treatments that can be applied to cheese crust)