It depends on how good you want to be is the short answer. A thoughtful answer takes a little longer.
Realistic Goals: Are your goals in tune with the rest of your life? What would you give up to reach them? Piano takes time, lots of it. For example to complete my ARCT in 2007, I started Grade 10 in 2003. Five thousand hours later I graduated! (This 5,000 hours included my theory studies as well as my piano lessons.) I gave up the music business to do this.
If you goals are more modest, aim for 2 hours a week of practice to start. Without practice, piano is no fun! The caveat is this. If you are a recreational piano student with the goals of learning some tunes and having some fun, skipping practice is no big deal. We can do it together at lessons, I will supervise.
Whatever the depth of your ambition, I suggest strongly that you schedule piano practice into your schedule.
On the subject of practice, another blog post for that.
You've got to learn your instrument. Then, you practice, practice, practice. And then, when you finally get up there on the bandstand, forget all that and just wail.
I'm a professional pianist and music educator in West Toronto Ontario. I'm also a devoted percussionist and drum teacher.