If I can remember an old saying correctly:
The hardest test to take is the second one (because most people drop out after their first test)
I agree that that probably around 90% (+/- 5%) of students never get past the first 6 to 12 months. Then they usually stick around until shodan, even if it takes a long time. After this, there seems to be another 90% drop off. That would put nidan at around 1% of entering students.