To get to BD the MLCer will have transversed denial and some of the anger stage and may have been in replay a while too. BD can come towards the end of replay when what they have so far encountered doesn't make them happy.
Often affairs have been ongoing for a long while before BD and therefore the spouse is not aware.
Whereever BD is replay after BD lasts at least 2 years in most cases and often 3 or 4. There are some early returners but they are a minority. Replay after BD takes a long long time
The stages and process can only been seen retrospectively with regards to when they happened etc when the MLCer has been through them.