It took a French man approximately 28 hours to reach World of WarCraft's new experience level cap.
After getting a copy of World of WarCraft's new expansion pack The Burning Crusade, "Gullerbone" and a bunch of his friends worked around the clock to gain 10 experience levels. Gullerbone is thought to be the first player to reach level 70.
You can read about how Gullerbone and his guild performed this feat here. And here.