I am someone who has talked about the 18 month mark, reading other sitches it seemed some people observed a change in behavior around that time, including those who got back with their spouses. That is, some, not all of course.
I noticed a change at this time (I am now 2 years post BD). H spent 18 months partying hard, that started to wind down about that time significantly. It was as if he had burnt out the energy he needed to burn out. He started to focus on his financial responsibilities for the first time and actively try to solve them about 18 months. His financial situation had caught up with him, and he needed to, but previously he had just ignored it. He also started to compliment me for the first time since BD, "the house is always clean, great outfit" etc. It was as if he took another look and was surprised to find I was not all bad as that had been him image of me for the last long long time. About 18 months was the furtherest H went from me emotionally, in that he seemed to be running prior to that, and stopped running so hard around that time. Replay continued, as did OW, it seemed to mark the end of the hard, active replay in a gradual transition. Those activities were not working anymore.
Focus also came back on his kids around the 18 month mark. Instead of just baby sitting them and having fun with them, he became more interested in parenting again. He started to recognise they had changes in their life too, and was prepared to work with me on issues such as anxiety in on of my kids etc. Previously, they were fine, nothing had affected them etc.
Two years on, H continues to try to pull his life together. He is still pushing for a divorce, which will be on the cards soon. He has not had any thoughts to getting back with me, I am in the friendly ex category. He is still in replay with the OW still present and that getting more serious, although they did seem to brake up for a short period a couple of months ago. I see H now starting to work on his big issues one at a time, his financial situation, on the right path now, his work situation, he has gone from trashing his reputation to getting positive feedback again. He still has other, deeper issues he has not addressed, and I do not believe he has taken ownership of what he has done, although he sees it from time to time.
So I did see change, movement through the tunnel, but the length of the tunnel and what will be on the other side of the tunnel remains unclear. This is an example of the two stages of replay some talk about, the hard first part, then the lower level, but still replay and avoidance of deep issues that is the second part. The length of this stage depends on the issues that need resolving, and the ability to do so.