When I used to play (waaaay back ago) we never got past 9th level, I had a fighter/mage 8/8 and a ranger who got up to 6th level. I also had other characters with other DM's, one of them reached 11th character and my DM (one of my players, I taught him how to play) decided he didn't want a high level player so he quit DMing...
As a DM I have three campaigns gooing on, two of them momentarely suspended sinve I've moved, but we play some summer days. In both of the the average party level is 9 and they have been running for four years...
The most recent campaign (only one year) is a 4th level average party...