Along with what the other two said. I keep my natural peanut butter in the fridge so it doesn't separate. Has nothing to do with longevity, which seems dubious to me because I've never seen peanut butter go bad. And in the past I've had an open jar of natural peanut butter for well over a year with no issues.
I just wait the 20 seconds for it to become spreadable. Though, you do have to only get a little bit at a time. You can pull out a hunk and expect it heat up that quickly.