Let me think...
...I started the last big campaign shortly after the Player's Handbook was released.... it ran for just over 2 years, with a couple of missed weeks. For sake of argument, let us call it 100 weeks.
We played one session per week, each session was between 4 and 6 hours. Again, for sake of argument, we will call it 5 hours per session.
So, the last big campaign I ran was 500 hours of game-play. In the end, I had eleven players, and a total of 14 characters (if you count Cohorts). The characters were, between 7th and 11th level. The 11th level character was one of two characters that managed to survive from the first adventure to the last (very climactic) adventure. This accounts for 10 leveling-ups.
So.. based upon this, 500 hours of play, 10 times leveling up = 50 hours per leveling up, on average.
Now... the typical adventuring party, according to WotC is 4 characters, I had about three times this. Thus, 50/3 = 17 hours. So that would be my answer...