That's impossible to tell. There are lots of factors involved (e.g., what kind of therapy, what kind of pathology is at work, etc.), some patients are now loquacious than others, and there are often silent moments.
One spoken minute is usually around 10 lines, so 6 minutes would be about 60 lines, or roughly 2 pages. Most transcription companies use single space, so this would be 1.5 pages at the most.
I think they are just supposed to make up a six-minute segment of a typical session. I just don't understand why they don't have the students do some roleplaying as patients and providers, record it, and then transcribe it.