How much you practise is connected with what you want to achieve. you won't become a proffesional if you do 20 minutes a day, not least because you won't have built up the stamina required.
If you're playing for your own pleasure, I can't see why you shouldn't play what you find fun, but if you want to imrpove then you DO need to do the boring stuff. I always found that rewarding myself for half an hour of tone exercises with something fun kept the mind working and stopped me wanting to throw the thing out the window.
When I was studying for my grade 8 exam I was playing around two to three hours per day on flute and the same on piano (i sat grade 8 on both in the same week)
