Being constant with your practicing is much more important than the total number of hours that you put into the instrument. It is also important to spread out the practice hours; a lot of the progress we make is done when we are away from the piano, processing everything we did.
A person that practices one hour a day is always going to make more progress than a person that doesn't practice for two or three days straight and then pulls an eight hour marathon session.
"If I miss one day of practice, I notice it. If I miss two days, the critics notice it. If I miss three days, the audience notices it."