Peanut butter really is one of the greatest inventions. It's an incredibly versatile food, perfect for desserts, sandwiches, healthy snacks, or just eating straight from the jar. And in addition to the nutrition value, peanut butter is always great for its shelf life and storage options.
In general, commercial peanut butter does not need to be refrigerated. Opened jars of your everyday, processed peanut butter can sit in a normal pantry for up to three months after opening. Keeping the peanut butter at room temperature also has the added bonus of making it more creamy and spreadable when you use it. However, if your jar of peanut butter is going to take longer than a few months to finish, or if you live in a climate that's particularly hot and/or humid, it doesn't hurt to pop the jar in the refrigerator. Once it's in the fridge, you've got about another three to four months to polish it off before it starts to lose freshness.
This timeframe, however, is for the processed, commercial peanut butter you'd find on a normal supermarket shelf. Natural peanut butter doesn't have some of the preservatives and hydrogenated oils that help keep commercial peanut butter fresh and creamy at room temperature for months. Because of this, natural peanut butter can really only be stored at room temperature for about a month before the natural oils begin to spoil. You can store your jar of natural peanut butter in the fridge and it'll be good for around six months. Just be sure to stir it before putting it in the fridge - if you leave the oils separated at the top, the peanut butter will solidify into one giant chunk.
As always, use common sense with determining if a jar of peanut butter is safe to eat. A simple sniff test will alert you if the peanut butter is going rancid or even just a little bit off. But as long as it seems fine and is within these timeframes, it should be good to eat.