How many hours is 50 to 60 WoW?

I'm curious to know, how many hours does it take to reach level 50 to 60 in World of Warcraft? Is it a significant amount of time or can it be achieved with a moderate amount of gameplay? And is there a way to speed up the process? I'm asking because I'm interested in the game and want to know what kind of time investment I'm looking at to reach those levels.