I see a quite strong correlation with student's age within the interval.
There is a formula, scarily accurate too.
Unfortunately I have forgotten it
Something like
time to solo (hours) = age (years) - 10
But the basic point is that the PPL training sausage machine's phychotic preoccupation with "going solo" is thoroughly unhealthy and wastes a lot of hours by stressing out the student through endless circuit bashing.