I'm not a pro, just a hobbyist, so I'm not sure my experience is relevant ...
My ideal practice day (which I manage better at some times than others, mostly thanks to my work schedule ... ) maxxes out at two hours a day for piano, with a little time left over to noodle on my other instruments. I feel like I make pretty good progress with that, though I could see how a bit more time could be helpful.
I doubt more than three hours a day would do much for me. I think my brain fills up at a certain point and then I'm just "chopping wood" (as I once heard someone describe useless practice). I've also wrestled with RSI since I was 18 (I'm ... a bit older now ... ) and have to be careful not to overdo it and have all that flare up.
During my brief time as a music major in college (upright bass, jazz and classical), the faculty had certain expectations for us in terms of practice. If you were a performance major, you were supposed to practice five hours a day; if you were non-performance (e.g., music education), it was three hours a day. This was only for your primary lessons, though. You were also expected to spend time on the ensemble music (everyone was in at least one ensemble, some folks were in two or three) and anything related to classes (e.g., everyone had to take some basic piano classes, and education majors took "tech" classes in the various instrument families). Plus go to classes, work your part-time job, etc. In reality, NO ONE ever got that much practice in on a regular basis. I'd guess it was closer to half that. Yet, most everyone managed to make serious progress and even graduate to respectable music careers.
One last anecdote. Joshua Bell visited and gave a talk while I was there. He was asked how much time he spent practicing, and he said "Not that much, only about three hours a day." Seemed to work out for him okay ...