Nice pic, what's all this complaining about, you look fine
"Excitotoxic injury cascades, active cell death pathways (apoptosis) and disruption of cellular energy regeneration" are, according to my massive book, all things which can occur "from minutes to weeks" following a TBI. They are obviously very complicated, but generally involve chemicals from the dead brain cells going into live cells and 'exciting' them to the extent that they too die, leading to a gradual cascade of cell death over time.
For me it took a week for my symptoms to fully kick in. I agree three months seems a bit of a long time, but since we've learned that alcohol can have a massive effect, it seems to me a pretty good guess that the drink got involved in this process and made its effects longer and worse.