Is a common question I receive. My answer is as follows.
Answer: I don’t know…
I think it depends on where you are currently at.
And moreover, I think it depends on what you actually call ‘retraining’ but, I think the answer should be as much time as you can REALLY spend time retraining and making sure that we are not repeating what is not working out.
This could be 5 seconds, 5 minutes or 5o minutes or 5 hours.
Sure, why not consider that a whole chunk of processing new information is happening while we are asleep... then we should also consider getting plentiful of rest after a good session part of our retraining/practice time.
What I know for sure that doesn’t work is spending endless amount of time obsessing over trying to correct the bad sensations and trying to discover more mistakes, or trying see if practice will make things better, just doesn’t work. I too, used to be very good at doing this.
Many times less is more.
We shouldn’t forget to put conscious effort into celebrating the small wins and keeping it a win by not practicing something that doesn’t work afterwards.
Short mindful practice is way better than long and reactive and non-strategical ones.
If you’re practicing a passage, or a pattern, I think it's better to do one good slow movement and call it successful and quit right away than doing another 9 more rounds of not so successful one after the initial successful one.
I think the mind keeps track of the ratio of occurrence of each movement, successful according to you or not. So you have to decide on which ones are the successful one by making that the predominant one in percentage.
When you play one successful passage and quit, this is 100% success rate. When you play one successful passage but had practiced 9 more not so successful one, the success rate drops to 10%, even though the number of times you’ve play successfully has not changed.
So you might only have to do so little to master something effectively.
So our teachers were right when they said, don’t practice wrong!
New habit creation is difficult because it requires a lot energy to practice and to repeat effectively to the point that it is automatic. Our mind’s default choice is the familiar than the non-familiar, regardless of it being a better choice or not.
If we keep on practicing a movement, desirable or undesirable, it will be the default pathway. So my advice to young students and seasoned musician with FD are the same. Spend as little time possible to achieve the desired ratio of success. If you can extend the time doing just that, even better.