Time on the water, not years, is a key factor.
Looking at @OB 's progress in '98 and '99 where he went from 15/32 to 22/34 and then 22/34 to 28/36 is enlightening. There's the old saying that it takes 5000-10000 repetitions to master a technique and it seems to hold true here as well. I figure that skiing 5 days a week for 8 months amounts to about 3060 passes, or 18360 turns. In each of those years he progressed 2 steps, 1 speed increase and 1 line length. So it would look like it took 9180 turns to progress each step. Those old Zen Masters were on to something.
At the rate that I ski, it would take me 3.5 years to progress a single step, which in my case are 2 mph speed increases at 15 off. I think I'm right on track. Oh sh1t, that means I've got another 8 years to go to reach 28 off.