Yesterday I got an email from a student wondering what she should be doing over the summer to maintain the work we'd been doing all year. She needed to know how much work to do so she didn't lose anything.
In my response to her, I said:
"As the body shuts down systems it doesn't use, in order not to regress you must practise
at least once every three days, though doing it more often will probably have better results. Then you're likely not to regress but to
progress over the summer. Not a bad goal, I'd say...
" Remember that several short practice sessions are better than a marathon session in any given day, though a minimum of 15 minutes is probably required for you to ever hit your stride. More than an hour at a time and fatigue starts to kick in. So make sure you get enough, but not too much..."
That seems like a fairly do-able guideline.