Depends on how you play them. I play the 3 card draw vegas way where it only goes through the deck 3 times. No way in hell 2/3rds of those can be solved.
I play every night on my Ipod before bed, a couple of games or so. Now, there are some times there are multiple plays in a game (taking one black six instead of the other). I always play where I take the card off of the deck with the highest amount of cards. In addition, I don't move my stacks to the top unless I feel it is necessary.
This post made me want to play. I just finished my first game of solitaire in at least 3 years. I won in 214 seconds. 3-card draw style, normal scoring.