How Much Should I Practice?

Good question.  I like to tell my students to play their favorite songs as a warm-up.  Playing piano should be a joy and something you look forward to doing, so play your best-est ;] ones first. Then, practice your assignment.  It depends on your level.  If you are a beginner to advanced beginner, try practicing your songs 3 -5 times in a row.  15 - 20 minutes should do it.  As I tell my students, if there is a trouble area...play those measures at least a hundred times in a row....well, you know what I mean... And then add it into your song and see how smooth it will play. The key is to play 5 days a week.  You will see progress this way.  If you have extra time, play anything.  Just play.  Make up a tune.  We will talk about that later.
I like these quotes from this website: www.pianoped.com
We should be taught not to wait for inspiration to start a thing. Action always generates inspiration. Inspiration seldom generates action. – Frank Tibolt
Inspiration is wonderful when it happens, but the writer must develop an approach for the rest of the time… The wait is simply too long. – Leonard Bernstein