A significant character in my story is a young man, late teens early twenties or so, who shortly before the story began had his mind wiped almost completely. All memories, all knowledge, and all skills more complicated than what an animal can do were completely wiped out of his mind permanently. Shortly after, he mutated into a supernatural creature, and since these mutations' exact forms are often in response to and in defense against the situations they happened in, one of the various supernatural abilities he obtained was a superhuman memory. While he doesn't get any of his old memories back, he now has perfect recall and unlimited storage space in his brain for new ones. Whatever he thinks or experiences, he can remember it flawlessly forever.
The main plot revolves around him being found by someone who, due to her own supernatural traits, can't be remembered by anyone at all... except for him. So she winds up taking on the duty of looking after him and keeping him out of trouble, while also trying to figure out where he's from and get him home.
But what I'm trying to get a sense of is just how much exposure to the English language this character's going to need to crack the code and start understanding it himself. We obviously have babies as a reference, but given that this guy has a basically-fully-developed mind and an infinitely superior ability to remember what he hears, obviously the process is going to be faster for him. But I need to get a better sense of how fast, and whether it would be realistic for him to start being able to speak in the middle of the cross-country road trip that constitutes the plot, or if I'm going to have to wait for sequels or a timeskip before this guy could realistically become verbal. This timeframe is crucial to know, because for various reasons, until he becomes verbal, I can't really add more members to the main cast besides these two.
How much exposure to human speech would it take for someone to learn a first language when they have no prior memories or knowledge, but the brain of an adult and a flawless memory?