There's no research that I'm aware of which provides what I think you are looking for, which is a clear guideline on how much practice per day is needed to provide what results.
But there is some research on the topic. The most general indication of the research results has been that there is a much stronger correlation between time spent practicing and results. This correlation seems to be real and present, but confirmation of other long-held beliefs, such as the importance of talent, has been lacking. Although there is also some evidence that suggests practicing earlier in life is worth more than practicing later in life, due to neuroplasticity. In addition, there is evidence that the quality of practice is underrated as well.
The commonly-mentioned (and sometimes disputed) rule of thumb, espoused by writers such as Malcolm Gladwell, is that it seems like 10,000 hours is a good measurement of the time it takes to master a skill, and that most people are unable to invest more than 4 or 5 hours of good practice per day. This jives with my own experiences enough that I view it as a useful guideline.
However, not everyone plans on mastering their instrument to that extent.
If you use this as a guideline, then the following results hold true:
30 Minutes Average Daily Practice:
1 Year: 182 hours | 10 Years: 1820 Hours | 50 Years: 9125 Hours
1 Hour Average Daily Practice
1 Year: 365 hours | 10 Years: 3650 | 25 Years: 9125 Hours
2 Hour Average Daily Practice
1 Year: 730 | 10 Years 7,300 | 15 Years: 10,950
3 Hour Average Daily Practice
1 Year: 1095 | 5 Years: 5475 | 10 Years: 10,950
4 Hour Average Daily Practice
1 Year: 1460 | 3 Years: 4380 | 7 Years: 10220
- 10,000 hours is an arbitrary target, but arbitrary targets are good.
- More is better. More than that is better too. Total practice time is probably the strongest factor correlating with instrumental proficiency.
- As you can see, you can scarcely have any chance of hitting this 10,000 hour target in your lifetime without at least 30 minutes of daily practice time.
- For the average casual learner in private instrument instruction, 30 minutes a day is a pretty good outcome. Few will practice more than this, and many will miss this target.
- The practice schedules which lead to faster outcomes also have a more exponential learning and growth benefit. That's because once you reach a certain level of playing ability, it's quite hard to advance meaningfully in the span of 30 minutes. In other words, there's a minimum amount of time needed just to reinforce what you know and prevent you from slipping backwards. And also there is a sort of economy of scale here when you practice alot because your focus is on music. Usually your tools, resources and supporting environment are stronger. You have put more attention on your equipment, you are more efficient at learning new things and using the existing resources. There are alot of benefits.
- As you can see, even with an extremely rigorous practice schedule, you can't possibly hope to do it overnight. No matter who you are, it will probably take a long time. For this reason, your approach should be realistic. You need to be able to invest lots of practice time over your life, despite demands from work, school and family. For this reason it is uncommon to meet people practicing more than an hour per day unless it's part of their job. 2 hours per day is a pretty virtuous practice schedule for the average working person.
- In high-pressure environments of aspiring professional musicians, you will hear people mention 8-hour daily practice schedules. However, this is usually pretty limited and short-term. There is also some research that has suggested there's an upper limit to how much good practice you can get in any given day, around 4-5 hours. Again everyone's different, but my experience among the more dedicated musicians I know is that 4 hours a day of average practice time approaches the upper limit that you can sustain for a long period and really get growth. Coincidentally this is a similar number to a few example Olympian training schedules I've seen.
Malcolm Gladwell, Outliers
Geoff Calvin, Talent is Overrated
There is more but if you read these books (which are sort of pop-social-science overviews), you should be able to find references to real research if you want to follow up on it.
However you slice it, this is a complicated process with many factors. The finding that the highest level of performers have invested a certain level of practice hours, or use certain methods in their practice, is a way for us to shape and guide our own objectives. It is by no means magic. However, even though we lack a clear formula it would be foolish not to take advantage of what information we do have, however fuzzy or anecdotal. The hard truth is if it means a lot to you, you had better practice alot.