Some studies have shown that 2 hours/day is about the optimum practice time, and 4 hours is the maximum useful time per day.
The "secret sauce" is very simple, but many people never figure it out for themselves: if you don't know before a practice session exactly what you want to achieve in that session, you are guaranteed to waste some of your practice time - and unless you get lucky, you will probably waste all of it.
Vague ambitions like "I want to play this piece more expressively" don't count as useful objectives.
Avoiding distractions and keeping mental focus are also important. Endless repetition of exercises or pieces with your mind wandering elsewhere is just another way to waste time.
Of course none of the above is specific to professional musicians. The same basic strategy applies to pretty much everything in life where there is an obvious difference between "high achievers" and the average Joe.
A couple of references:
- Duke, R. A., Simmons, A. L., & Cash, C. D. (2009). It's not how much; it's how: Characteristics of practice behavior and retention of performance skills. Journal of Research in Music Education, 56(4), 310-321.
- Ericsson, K. A., Krampe, R. T., & Tesch-Römer, C. (1993). The role of deliberate practice in the acquisition of expert performance. Psychological Review, 100(3), 363-406.
Here's a quote from organist Kevin Bowyer's preface to his edition of Sorabji's three Organ Symphonies (the shortest of which has a duration for about 2 hours, and the longest more than 8 hours - all at the extreme limits of virtuosity).
The only way to attain mastery of these huge works is to plan the
process of learning far in advance of a performance, perhaps two
years or even more.
(My emphasis) - and by "plan" he means a detailed timetable, broken down hour by hour, of every daily practice session over that time scale.
And that level of planning is from someone who, as a conservatoire student, didn't realize that his teacher had given him a list of works to study over the whole of his first year, and learned them all within the first week!