RE: How many years do you stay at each class and why
I moved out of Sportsman after 2 short seasons, mostly because I was getting bored with it. Still had plenty to learn there though. I tried practicing Intermediate a few times during my last year flying Sportsman. It was a big challenge at first, but I found that I was making improvements really quickly. I think what pushed me into Intermediate was the rate at which I was getting comfortable with it.
Now there is really a lot for me to learn - timing, wind corrections, and more opportunities for rudder corrections.
I'm flying less these years with a new job and a committed relationship with a wonderful woman, so looking at maybe 3 contests a year (2-4). At that rate I imagine I'll be in this class for 3-4 years, at least.