The perennial question. The snappy answer, "how good do you want him/here to be?" The thoughtful answer is, "it depends". Here are my some thoughts before I answer the question.
1. No practice, no progress. No progress, no fun. No fun, no lessons.
2. Most students love to play, hate to practice.
3. Children on sport teams are not left alone to practice. They have coaches. Your child needs you as a coach. Normal children cannot be left alone to practice.
4. Students need to have support from both parents that this "piano thing" is a good thing. If sports and other extra-curricular activities take precedent over music, this piano adventure is likely doomed.
5. Students rarely ask to play music unfamiliar to them. If you wish to have your child learn a particular style of music, hearing music around the house and concert going is a must. Students need to be emotionally connected to what they are learning.
6. A student who learns to associate practice as a punitive activity will quit as soon as they can wear you down. A good example is, "you have to practice for 20 minutes before you can go and do something really fun speech".
7. Piano lessons is not a great activity for lazy students to learn discipline. I'm not a drill sergeant.
Now for some positive thoughts.
1. The first goal of practice is to be prepared for the next lesson. That means the student can at least play the piece or pieces is a slow steady manner. The written work is also done. Scales and chords are like push-ups. A few reps won’t cut it. If you sit with them and help them in an enthusiastic team spirit complete the work everyone wins: you, me, and them.
2. A student who connects emotionally with the music doesn’t count the minutes, they are too busy having fun. If your child is not having fun, this may be the problem. Countless children learn online every day from their peers on YouTube. They self-select what they want to learn. Nobody criticizes them. They can learn just the “good bits”. They start and stop when they want. They are learning, and having fun. Many piano students are still expected to sit by themselves, organize a long list of learning activities for maximum productivity and then systematically execute. Let’s think about that for a minute. Most adults can’t do this. That’s why there are managers at work. Sitting by yourself, looking at a dry list of teacher commands, for a specific period of time comes from the Victorian era, an attitude which assumes children are passive objects who can be directed to a specific outcome. That won’t work in 2015.
To sum up.
1. Help your children.
2. Sit with them.
3. Participate in recitals and festivals. They are the hockey tournaments of the piano world.
4. Be patient.
5. Be enthusiastic.
6. Listen to music, your preferences will be their preferences in the early years.
7. Play yourself.
8. Make music a priority in your life and the lives of your children.
9. Let them play at the piano: improvising and fooling around.
10. Let them learn off YouTube and play music on their electronics. It is really fun.
11. If you can, read a book on teaching piano. A modern book written in the last 10 years. It will explain everything in greater detail.
Beginners: 10 minutes a day. Add 10 minutes a day for each year of lessons up to about 1.5 hours a day for Grade 9 level.