Not very accurate is the easy answer.
Who of us remembers every word of a conversation that happened twenty years ago? or even last week? It's especially difficult when we hear something and only later—perhaps a decade passes—before we realize the significance of what someone said.
Most readers are smart enough to figure out that dialog isn't word-for-word accuracy; however, they assume the author strives to be as close to the truth as possible.
When I write others' autobiographies, I ask my clients to focus on their feelings and their emotional responses. How they felt is as important as the literal words.
* "How did you feel when you faced her?"
* "How did you respond when he yelled at you?"
* "Did it hurt your feelings when your father . . . ?"
* "How difficult was it for you to ask her to forgive you?"
My dialog in nonfiction may not be accurate,
but I strive to be as truthful as possible.