Here is a recent case of 10 years.
Here’s one of 19 years. I saw a documentary about this chap on one of the cable channels. His speech is still pretty slurred, but intelligible if you’re around him enough. What’s interesting about this particular case is that he apparently has no capacity for learning anything new. His mind is still stuck where it was when he had the accident. His daughter is still a baby, and he seemed to think his (now adult) daughter was his wife. She mentioned how much it hurt her because he keeps talking dirty to her and using the “F” word a lot. They would tell him things, but he doesn’t retain anything.