I recently played in a 9 month campaign where we had a 4 hour session once a week and only took a month long break in December and we reached level 22 without any of the levels given to us. I am also 1 year into running a 3 year campaign (3 years only because thats how long i will be at the SDSMT) and so far they have achieved level 11, I think, we are having a break for summer and get to continue when school starts up again. We have a session every week that will run anywhere from 6-12 hours straight, thats how I was raised, and by christmass my players will have reached level 35 (about) and they will have earned every point of exp. However, I run my players very hard with the 12 hour sessions and have desiegned my plot around them being high enough to run armies and have massive battles against ultimate evil after they have reached level 35. They will still gain exp. at about the same rate but luckily for me the amount needed to level increases dramatically. That being said I fully expect them to reach, and surpass, level 40 on there journey to "save the world" (my players are begining to hate that line). So, I guess the answer to your question is yes, but rarely. Oh, and according to my players they cant wait till we start again so I guess they are haveing fun reaching new levels of character development and character level that they have previously been unable to attain.
P.S. Sorry for any spelling errors.
Just another DM
Ive been gaming so long I cant tell if this world is real or just another of the ones floating through my head.