It's very dependant really, for first timers, the sheer ball busting difficulty can ramp you up to about 20+ hours of gameplay, many of those will be spent throwing your controller across your room though, but if you're used to the gameplay design, it can take you anywhere from 10-15 hours.
Again, it's also dependant on difficult, it took me about 11 hours on Path of the Warrior difficulty, and it took me even longer on Mentor, Master though...I never finished personally, my balls aren't quite big enough to take that on just yet, still, like I said, the gameplay on initial difficulty such as Warrior is very much dependant upon your skill level, if you're used to it, about 10-15 hours, if not, maybe 15+.