This is a difficult question, because it depends on how far you have progressed and what you want to achieve. For beginner piano student, a minimum of 30 minutes of practice a day is recommended to make progress and prepare for the lessons. Practice more and you will get more done. A correct and concentrated way of practicing is very important, and a lot of attention is given to this in the piano lessons. Advisable it is to practice at regular times during the day. Practicing can be a lot of fun if you experience progress, so keep up the regular practice. It may sound contradictory, but the further someone has progressed, the more time it takes to learn a new musical piece, because the works get often more difficult. A beginner will be able to learn a few new songs in a week’s time in only 30 minutes of practice a day, but for a professional it can take several months for learning a new work by Liszt or Rachmaninov.
If you have the aspiration for entering the conservatory, 2 to 6 hours of practice, depending of how much other school work you do, is the norm. Professional pianists study at least 4 to 6 hours a day. When I was still a student at the conservatory, 6 or 8 hours practice a day was like a normal day. For a professional pianist, it may take for example 100 up to 150 hours to learn a new Beethoven sonata very well. This music is difficult, deep and complex, a lot of attention is paid to musical details. Yet easier pieces can be sight read from sheet without too much effort.
Learning to play the piano takes not weeks or months, but takes years. But from my experience, and I’m sure many pianists experience, I can tell, it is never boring, always interesting and fun to learn more and more!