I want to say a lot here, but I have made a promise for only good vibes on Election Day. So here is my experience.
If the time between “neck pour” and 2nd pour is a week, or even a month, you might notice a slight difference. 3 month, 6 month, longer… you will 100% notice a difference. I have done this many times.
Air time is real. On every toasted or double oaked bottle that I’ve ever had, the first pour was fine. The first 1/3 of the bottle was ok. The middle 1/3 of the bottle was good. And the last 1/3 was excellent. I would guess my bottles are open (start to finish) anywhere from 6 months to 2 years. This is on every toasted or double oaked, well over 40 bottles at this point.
Now on “standard” bourbon, I have certainly noticed as well but not as consistently. Air time definitely changes the whiskey. See below as well.