My kids at 4.0 and 3.75 years old started playing at about 15 minutes a day. It was tough to get my son to sit still initially. We would let him take run-around-the-house breaks. My daughter didn't have and still doesn't have a problem. Now at 5.75 and 4.2, they play about 30-40 minutes and 20-30 minutes, respectively. It's a bit of a struggle to get my DS5 to play that long, but the teacher advised to break it up into 2 shorter sessions. One issue is that the better the kids play, the more the piano teacher wants them to practice. The good news is that both kids will go to the piano on their own and improvise and mess around. And with more lessons, the better the improvisation sounds.
