Once you open a jar of peanut butter the opened jar of peanut butter will last up to 4 months before it loses it's freshness.
The peanut butter will remain safe to eat after 4 months but it won't taste as good as it would if you eat it before the 4 months.
If you want to keep the open jar of peanut butter fresh longer than 4 months then you should store the opened jar of peanut butter in the refrigerator.
By refrigerating the opened jar of peanut butter you will extend the life and freshness of the peanut butter by another 4 months.
So in the refrigerator the opened jar of peanut butter will last for up to 8 to 12 months before it goes bad.
Unopened jars of peanut butter will last a few years or longer before it goes bad but it will still usually be safe to eat as long as it doesn't have any mold or anything on it.
I've kept unopened jars of peanut butter in my pantry for 5 years before and although it wasn't as tasty the peanut butter was still very safe to eat.
I used the older peanut butter to make peanut butter cookies with.