I think of it like the time period where the WS is still in the A mindset, and they're experiencing distorted thinking and emotional confusion.
I've read so much on it and some WSs think of it as part of limerance and blindness to see beyond that but my fWS never experienced limerance.
He didn't love and certainly in a lot of ways didn't even like the AP, but his head was in some kind of fog for sure.
He romanticised the A as a great friendship, even though this woman was ostensibly a psycho.
He lost all moral clarity surrounding that - such as feeling entitled to miss his "friend" or to offer her comfort
He lost all integrity, and lied to me about NC and more.
He was completely irrational. This person was quite literally doing things like threatening him or trying to destroy his career and he couldn't see it fully.
He was very much addicted to what the A gave him. I think this is different for every WS but in my fWS it was the belief in being adored, so amazing someone would try to kill for him, a sense of "coolness" and this kind of love bombing that was completely unnatural.
It took him about 4 months to move past that once AP was no longer physically present, and then I think another year to truly see how nuts he was.
I do see that "fog" as a real thing but rather than being an excuse I just think it's the place WSs end up after a series of decisions turns then into a shit person.
I saw it as addiction, not so much to the A, but more like when people let themselves slide to rock bottom and they're not a person of integrity anymore.
Having an A and sustaining one takes a lot of bad, selfish and destructive choices and the shock of dday isn't always enough to snap them out of it.
I think the answer to "how long" is up to you, because in that destructive, selfish mindset I think they will keep going as long as you let them.
The "fog" is I think also contagious because the blindsided and traumatised BS is in so much shock and pain that they also can't act rationally or often fully act in ways that are best for them.
I second the advice of the 180 for you. If you can get yourself out of the fog, you will start to find your anger and your self worth. Many of us make the mistake you're making to deferring degrees.
My fWS and I are very happily reconciled and I am so happy I stayed but I also deeply regret that I didn't tell him to go f himself much earlier.