I've had a varied relationship with practice. When I was younger, I practiced a lot more because I had yet to develop my own practice frameworks and methods. These days I've gotten to know myself better and so when faced with that question I often reply with "I practice as much as necessary".

I will say, the most I practiced in a day was 10 hours. Definitely not advisable over a long period of time and also dependant on your instrument. This was a special few weeks when I was competing at the Queen Elisabeth competition. I was also 20 years old - pretty sure if I attempted this now my neck would break.