As with many things there is no short answer that is straightforward correct. Therefore I will firstly give you a short answer in an as simple way as possible and then explain what is under the top of the iceberg.
I would say that each individual is different and the concentration skill of every person differs a lot. So there is no fixed time that I (or anyone else) can really suggest. My advice would be choosing a minimum practice time on a daily basis and following it as regularly as possible. (If you sometimes skip a few days it is completely OK. Even the top pianists do it). If to say out any amounts at all I think the very minimum would be half an hour. But now to the more interesting part where I will explain how you can choose your best practice time.
First you need to know what is practice as such. Practicing can be defined as a process where we create new habits and change old ones. As the idea of practice is improvement the aim is to create good habits not bad ones. And here starts the important part for you.
Basically it can be said that the whole sum of your piano playing habits is what you are as a pianist. So the habits define how you play (of course the psychology and different aspects of piano playing are much broader). But creating good habits has some preconditions. I will point them out.
1) Find Interesting Short Term Goals With Every Bit of Practice*
When you do not have a clear goal you can you never be sure that what you do is effective and works. So goal, a mental image of what you want to achieve is extremely important. It is a very common trap for beginning pianists to just sit and play instead of practicing. Then “practiced” can be marked for a day but actually nothing really improved. Just playing how it comes is often actually worse than not practicing because while playing unawarely lot of random and instinctive habits are formed. Instinctive habits are ineffective in nine cases out of ten. If you have a short term goal your practice has a meaning, a direction.
* All short term goals will be given in a friendly and simple manner in my piano courses.
2) Being Concentrated Enough Helps to Practice With Good Quality
If you have a clear goal but your thoughts are wandering, it will not make any sense. Then you are playing with little contact with the actual reality of playing and it is not possible to adapt new good information and reinforce good habits. But when you concentrate and focus well, it is very likely that you can progress, master new skills and techniques, form good habits and replace old (bad) ones.
To concentrate for a relatively long period of time is not easy. Sharp concentration of beginner pianists usually lasts for thirty till forty five minutes (in some cases longer). Then brain needs a rest. When you get used to being focused your abilities get better in time. Also, focusing skill will flourish in all fields of your life. But what next? Can you practice more?
3) Practicing in Chunks
Of course. Practicing in chunks is very effective. After the first set of practice you can make a rest what can basically mean doing something completely different. It is not exactly rest what your brain needs, but other brain centers need to take over and work. After a while, sometimes even after fifteen or twenty minutes you can return to the piano refreshed. But the most important is to evaluate the situation adequately. You should stop instantly when you are feeling fatigue or the concentration fades. And your adequate decision and observation will decide your optimal length of one practice set. This set should maybe be the minimum time of practice per day.
4) Try to Repeat Things Over – It Saves You Very Much Time
What is also very important to bring out the importance of repetition. The importance of repetition can be explained with the concept from evolutionary psychology. According to this our mind creates a hierarchy of surrounding events based on their occurring frequency. Things that happen more often will be higher in this list and the less something reoccurs the faster your memory will delete it.
Actually forgetting is not an inability of our brain. Forgetting is a certain feature of our brain to simplify problem solving. It can be said that brain intentionally deletes information to filter and reduce the amount of information that needs processing and is surrounding us. The only thing that protects against this “violent” deleting is repeating over and over again.
Research shows that when we repeat things the time length we remember it without repeating it grows exponentially. For example in the beginning you need to repeat something twice a day, then once a day, then in two days, four days, a week, in two weeks, in a month etc. Soon you will remember what you learned for years.
So this principle of repetition should be also kept in mind when you want to practice efficiently. When something new is learned and you do not repeat it over in twenty four hours about seventy five percent of the learned information will be lost. Therefore having two sessions per day where one is a shorter session of repeating over what you practiced earlier is really efficient.
For the conclusion of this article I will bring out some points that could work as a plan for choosing your best time of practicing on a daily basis.
1) Decide how much spare time you would like/can contribute to piano playing on a daily basis
2) Find out how long is your optimal session length according to concentration skills
3) Try to find time to repeat everything you learned over later on the same day to double or even triple the effectiveness of your learning.
4) Enjoy a good step-by-step improvement process built up from clear short term goals
Thank you for reading. I hope it gave you some insight. I also invite you to join the free one month piano course for adult beginners. If you have any ideas you want to share or ask more questions please feel free to contact me.
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