Hours of practice vary for me. Anywhere from 1/2 hour to marathon days of eight hours.
I warm up with scales and various exercises, read through scores I have no intention of learning - for sight reading practice, and in order to get familiar with a composer's methods, play some favorite pieces from past months or years so they're not entirely forgotten, and then spend the majority of the time on learning new pieces.
Regardless of how much you practice, it's important to make good use of your time. I'm a big proponent of slow practice.
I believe it was Confucius who said 'Practice slowly, play quickly'.
And be sure to take some time to just have fun at the piano. Improvise, record yourself, learn some piano duets so that you can play with other musicians.
So much music, so little time!